How to sync/upload a "Casual walking" or "Speed walking" activity into Garmin Connect? And: Why do some activities get synced with the correct name and others don't?

The name and the type of the activity are two completely separate properties with no fixed relation.

The type of the activity is hardcoded in the FIT file on the watch. Some models offer a more complete choice of activity profiles available for the selection, others have the choice quite limited. So if you create a new activity profile from the scratch, it will be listed in the category Other, with no specific activity type assigned, and the Activity Profile name you enter on your watch, will be used for activities recorded with that profile (partially depending also on the settings explained further below).

When you use the function Copy Activity for creating the custom activity profile, the original type of the activity will be used for yours too. The name it will be recorded under, depends on your choice of the setting Activity Name in User Settings of Garmin Connect Web:

• Workout Name - the activity will use the location and the name of the Workout you executed (it may be a standalone workout or an individual workout of a Training Plan). For example: London - Benchmark Run. If no specific workout or training plan was done during the activity, the next pattern will be used:
  
  
• Location & Type - the activity will use the location and the type of the activity you selected (or the type of the activity you copied it from). For example Saint-Tropez Open Water Swimming
  
  
• Location & Device Activity Name - the activity will use the location and the name you used when creating the new Activity Profile on your watch (this is probably what you are looking for). For example: Prague Underwater Rugby.
  
  
• Untitled - the title will simply be "Untitled" (a little bit useless option, I think)

If you want to assign some activity types not available on your watch, you may check out the Connect IQ store. My Garmin Instinct does not support the CIQ technology, hence I have no personal experience with it, but know that many Garmin watch owners use for example the app Any Activity when the desired one was not available on their watch.

Thanks for this clarifying answer! Sounds great. However, it still doesn't work. I followed your instructions, changed the settings to "Location & Device Activity Name", and created an activity called "Casual walking" but it still does not get assigned into the "Casual walking" activity category. The name is now, indeed, correctly taken from the watch ("Casual walking") but it still appears in the "Walking" category instead of the "Casual walking" sub-category. Any solutions for this? Or is the "Any Activity" app needed for this purpose?

and created an activity called "Casual walking" but it still does not get assigned into the "Casual walking" activity category.

That's exactly what I wrote, so please re-read the post. The name of the activity has nothing to do with the type of the activity. The type is hardcoded in the FIT definition file of given sport, and will remain identical even if you copy it while creating a new custom activity profile. The only way to change the type, would be hacking the FIT file with a tool like the Fit File Repair Tool. Using the recommended Connect IQ app may be another option for achieving it (though as I wrote, I cannot confirm it, since I cannot install CIQ apps on my Instinct).

... BTW, the FIT files holding the definitions of the Activity Profiles (both the built-in and the custom ones), are stored in the folder //GARMIN/Sports on the device, if you decide hacking them

Just a few words about the difference between "Casual walking" and "Speed walking". It influences the calculation of calories burned during the activity.

For example today I walked 17.14 km in 172 minutes on max speed (6 km/h) on my walking treadmill. When I specified "Speed walking", the "Calories" field was assigned value 1135. But when I changed activity type to "Casual walking", the value was changed to 681.

Two things to note:

1) My treadmill (Xiaomy WalkingPad C1) measured 1204 Calories for this activity.

2) According to Wikipedia speed walking is 7 to 9 km/h.

Yurii said:

Just a few words about the difference between "Casual walking" and "Speed walking". It influences the calculation of calories burned during the activity.

As long as you record the activity with a device with a HR monitor, the Calories are calculated using the heart rate, and the heart rate variabilty. The type of the activity you choose has no impact. I have just tested it again, just to be sure - changing a walking activity from casual to speed walking and vice versa makes absolutely no difference - the Calories remain the same.

Yes, you are right. I meant manually added activities, not the activities recorded via fitness devices. Sorry for the confusion.

These days I work from home using standing desk + walking treadmill. It is not practically possible to accurately record such "walking" via my Forerunner 935, so I just add the activity manually at the end of the day using data from the treadmill.

In this case there is a difference mentioned above between "Casual walking" and "Speed walking".

Yurii said:

It is not practically possible to accurately record such "walking" via my Forerunner 935

Why not? If the distance is not exact, either recalibrate the stride, or simpler - just edit the distance of the activity, entering the distance from the treadmill. Much better than entering all data manually, because you'll have the HR, pace, and cadence graphs, among others.

I'd use the walking indoor, pausing the activity when I stop, use the option Resume Later that allows you to use the watch normally during the pauses, and at the end of the day I would enter the distance from the treadmill in Garmin Connect.