Why does my 530 want to update my FTP with a number less than my FTP?

Mate...just disable the auto detect function already...that’s your answer, don’t want it to prompt, disable the function. 

Ignoring a design problem isn't a solution.

Andrew Van Til said:

The only condition where it shouldn't is if I explicitly do a FTP on the device. In every other scenario, it can only reliably guess upwards. Any downward guess is going to be wrong because it doesn't know the reason why the power dropped.

If I understand you correctly, the design problem is that the Edge will take your best 20 minute power, and take 95% of this to be your FTP, even if you have deliberately chosen to ride below threshold???

It's not a design problem.  It is a feature people may or may not find useful.  If you don't find it useful turn it off.  There are many features on these devices that I have no interest in.  I just don't use those ones and don't complain about them being there.

If you prefer to do actual FTP tests to determine your FTP just do the tests and don't use the calculation.

mcalista said:

If I understand you correctly, the design problem is that the Edge will take your best 20 minute power, and take 95% of this to be your FTP, even if you have deliberately chosen to ride below threshold???

The FTP calculation doesn't simply do that.  Firstbeat (who are the company behind this calculation, not Garmin) don't say exactly what the calculation does but they do say on their website that it uses their V02 max calculations and heartbeat data.

I don't know if that's the algorithm it's using, but in concept, yes.

There are probably some other conditions that must be met, like HR at some level for a sustained period. I get the prompt most reliably after doing an extended SST session.