Confused about training status / load.

Probably the advice you do not want to hear but is the most on point here: Worry less about these numbers, make sure you do 4 easy rides before you do one hard one, and enjoy it :). Use these numbers more as a reality check to see if something might be very off (e.g. you only ride easy, or way too often hard), just to get an idea that you might want to check your approach. Using these numbers to guide the training itself is not what I would recommend doing, as they contain a lot of maths built on guestimates: You can do better by knowing your HR Zones and logging your work-outs according to HR Zones and RPE.

More to your question about the numbers: I would assume the mathematical magic behind it is using some kind of time-weighted approach: The more recent the work-out is, the more it effects the numbers, while older ones are less relevant - it seems that once you add a new work-out that shifts the calculation, maybe dropping some older work-out completly out of calculation or at least weighting them with less weight (because they are old). So, I would expect your anaerobic sank because your recent work-outs contained less anaerobic work than your previous work-outs that are now being considered as less impactful.

Yes, you can use the heart-rate gauge as one of the data screens, that's what I do.

If you in the light blue, that's low aerobic, etc. Unfortunately, that is new as of 3.30 and not doculented yet.

There are also two data fields that you can add that show the impact of the current activity on Aerobic and Anaerobic performance: https://www8.garmin.com/manuals/webhelp/forerunner945/EN-US/GUID-5176B6E4-F272-4437-BDA7-85B31231CB3E.html

See also this: https://www8.garmin.com/manuals/webhelp/forerunner945/EN-US/GUID-A8716C0B-B267-4C42-B45F-B9C7928BCA19.html

Notice that your excerise load is calculated from October 20 to November 17 (as of the image you provided), or about 30 days. Most likely you had a ride fall off the front end of the 30 day period and is therefore no longer being counted in your load total. If it is a comparatively big/hard training event that is now outside the calculation period it can greatly affect your load totals.

As Fredderic said, a new activity will cause the rolling average to move forward so that the new  exercise load is a function of both the new activity that is added and the older activity that has rolled out of the moving average calculation. 

Thanks for the replies

From one of @HermanB posts it seems that filling up the Anaerobic bar is not simply a matter of spending time in zone 4 / 5. The "analytic algorithms" picks up certain "spikes" in effort and allocates it accordingly. 

So (if I'm right) it's NOT like this:
Zone 1 + 2 = Low Aerobic
Zone 3 = High Aerobic
Zone 4 + 5 = Anaerobic

Dunno if @HermanB is still around to confirm?

"Using these numbers to guide the training itself is not what I would recommend doing, as they contain a lot of maths built on guesstimates: You can do better by knowing your HR Zones and logging your work-outs according to HR Zones and RPE."

Hopefully @HermanB (or someone else from Garmin / Firstbeat) can join the discussion, but this makes zero sense to me.

That's what I did BEFORE I got a top end Garmin device, i.e. logging time in HR Zones and eyeballing it from there.
The promise of these devices and algorithms is to get personalised feedback so as to inform my training decisions.
See here:
"The ability to quantify the activities in terms of the amount of strain placed on your body opens the possibility of truly personalized training guidance and feedback."

opens the possibility of truly personalized training

And that's where it is, a possibility. The algorithms are all based on averages so will/can never be truly personal. Use them for guidance not prescription.

Note also the chronic training load is a rolling number where high loads will have a diminishing effect as time progresses. The lefthand edge of the training load suggests an elevated training load that was decreasing in subsequent weeks.

For anaerobic you need quick rises of your HR, not necessarily zone 5, or as FirstBeat states: "Large, sudden increases in intensity". Zones are always arbitrary, so they can not be set right or accurate. Better would be percentage of HRmax instead of zones.