why does my zumo 396 calculate a route that's been calculated by Base Camp?

Are you sure maps are the same? A while back a map dropped the last 0 from its name, can't recall if it was in BaseCamp or on a device, but that caused the same problem. Check both map names are exactly the same.

Absolutely sure.  And the first of the two routes that I imported didn't recalculate, only the second.

I know that you said that you were absolutely sure, but FWIW, I was sure, too and I missed something until Sussamb just mentioned it. I used Express to copy the maps on my new Zumo XT2 to my PC for use in Basecamp. In Basecamp, the map is City Navigator North America NT2024.1. On the Zumo, the "same"map - the source that I copied from, is City Navigator NA NT2024.10. So, when I connect the Zumo to BC, I get a message saying "Routes sent to zumo xt2 were recalculated beause they were created on a map that is not installed on that device."

Since there does not appear to be any way to edit the name of the maps on either the zumo or BC, I'm going to assume that the re-calculation will not cause me any grief.

You can use JaVaWa's GMTK and Device Manager programs to edit the names.

"City Navigator North America NT2024.1" is the same as "City Navigator NA NT2024.10". It's not unusual that the name on the device slightly differ from the name in BaseCamp.

Thank you. That's good to know. I am having endless problems getting BC and a Zumo XT2 to play nice, but I don' believe they are due to map naming issues. Of course, Garmin solution is simply to tell me to use Tread, which is crap.

Thank you

So I'm guessing no one else has this issue?

I've had and continue to have this issue. I've gone back and forth with Garmin for the better part of a week. Their tech told me that this is "by design". Garmin does not support the use of Basecamp with the XT2. They do not support the use of Garmin Explore with the XT2, either. They want to force us to create routes in the Tread app or on the device itself. Problems using any other applications are "by design"

I suspect that all routes are recalculated, however, it is only noticeable on longer or complicated routes.  There are a number of reasons why this might be desirable.

Any deviation from a planned route will cause a recalculation.  It doesn't matter if it's an unexpected detour, pull off to a rest area, to get gas, food or decide, on the spur of the moment, to see the world's biggest ball of twine.  At this point, you don't want the device to change your intended route.  In this regard, all possible uses of a device must be considered.  If you were visiting an unfamiliar large city and ran into a road closure, having the device recalculate a route is a very desirable feature.

There is no guarantee that your device and BaseCamp will produce the exact same route with the points provided.  Your computer and device differ in available memory, processing power, cpu and probably coding language.  Neither device nor computer investigates all possible routes, they must limit and trim possible routes as they go to arrive at the final route.

The map used to create a route may differ from the one used to navigate the route.  For example, saving a route to the home of a family member..  Some people probably keep some routes on their device all the time.

There is also a memory limitation on devices that isn't present on computers.  It is more efficient to save only the points needed to calculate a route than all of the points in a calculated route.

It is a wise practice to check routes on the device prior to navigating them.  It doesn't matter whether the routes were created on the device or uploaded to the device.