I read the white paper on Firstbeat's site (Garmin licenses the software from Firstbeat) but there's nothing relevant in there.

I'm pretty sure my FTP has gone up enough that I need to test it. I got an IF of 1.164 last night doing hill repeats, it was tough but not "one for the books." There are other things that tell me I'm getting stronger, like RPE, and the fact that I'm using smaller cogs compared to a few months ago.

My weight hasn't changed much in months. HR is harder to get a handle on but the max HR I achieve in each ride has gone down slightly over the past year.

My Garmin says my cycling VO2max has been going down, at the same time it says my running VO2max has been generally rising.

I thought it was measuring HR/watts as a function of body weight, so if my power is going up and my weight hasn't changed, I don't understand what's going on.