Garbage in, garbage out. Check your settings.

Do some more reading about Garmin and Firstbeat.

Then post a lot more information if you want sensible help.

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Trust me I’m a doctor!
Well, I have a PhD :-)

Anaerobic Training Effect generally requires short hard intervals above threshold pace. Steady state efforts (even if they are fast) are generally aerobic in nature.

The aerobic / anaerobic effect calculations are based on your settings (Garmin, Polar, .....) :
HR zone setting, for a given sport (run, swim, bike, ...)
Power zone setting, for bike (if you have power meter)

If you put the correct zones for you, for this sport, you will have a correct calculation.