Anaerobic Results Don't Make Sense - Lower Anaerobic for Higher Effort?

I'm not an expert at this stuff, but I think anaerobic training effect is based on doing high-intensity intervals, not just spending lots of time in your top HR zone.

https://www8.garmin.com/manuals/webhelp/forerunner935/EN-US/GUID-7275629E-743A-4658-A284-C84F42A66AE5.html

Aerobic Training Effect uses your heart rate to measure how the accumulated intensity of an exercise affects your aerobic fitness and indicates if the workout had a maintaining or improving effect on your fitness level. Your EPOC accumulated during exercise is mapped to a range of values that account for your fitness level and training habits. Steady workouts at moderate effort or workouts involving longer intervals (>180 sec) have a positive impact on your aerobic metabolism and result in an improved aerobic Training Effect.

Anaerobic Training Effect uses heart rate and speed (or power) to determine how a workout affects your ability to perform at very high intensity. You receive a value based on the anaerobic contribution to EPOC and the type of activity. Repeated high-intensity intervals of 10 to 120 seconds have a highly beneficial impact on your anaerobic capability and result in an improved anaerobic Training Effect.

https://www.firstbeat.com/en/anaerobic-training-effectworkout-explanations/

There's a FirstBeat employee who used to post in these forums. Idk if he's still around. HermanB?

Anecdotally, if I do strides or fartleks during an otherwise moderate training run, I do notice a higher anaerobic TE compared to not doing strides and fartleks.

Thanks for these. The thing is that although I didn't consciously do fartleks or strides, I DID do more high-intensity running in the second run, the one that gave me a much lower anaerobic training effect. I'm beginning to wonder whether the explanation is that the first run raised my baseline, making it harder for me to get more anaerobic training effect 48 hours later... someone must know all this; I'm surprised at how little offical info there is from @Garmin on this. 

Anaerobic runs are typically less than 2 minutes, any sustained run eg 5K is going to be aerobic. I can typically get my anaerobic score up only when at the track doing intervals. The shorter and more intense (eg 200s/400s/600s) the better and you need to let your heart rate come down between intervals. Also, if you just started training with your watch, it is still figuring out your thresholds, normal training loads, max heart rate, etc, which are all part of the formula.

Thanks. It just seems weird that the exact same 5k run but at a much higher real effort produced much less anaerobic than the one at a lower effort (I didn't do intervals etc. with the first one either). 

Exactly the same