Switching to Assioma Duos from Stages L-only saw my FTP take a hit. Part of the issue was a slight, but consistent power mismatch between stages and the Assiomas (L only vs L only).
However, the main issue was that the Duos confirmed what I had long suspected - I had a LR imbalance. Turns out it was in the order of ~55/45% in favour of my left leg. Not huge, but enough to have a significant impact on my FTP value measured by the Stages.
I did some trials with the Assiomas and Stages running concurrently and plugged the data into DCRainmakers analysis tool. As an example, let’s say my Assiomas were reporting 300w (165w left + 135w right), at the same time, my stages would be reporting ~330w (2*165w left).
Of course, if someone had a weaker left leg, then Stages would under report and you’d probably see a free FTP bump with the Assiomas.
Having the LR data and being aware of my imbalance has actually helped me to improve it slightly. I don’t ever expect it to get to 50/50, but I’ll take any improvements.
There is a bunch of other data available from the Assiomas and it is very easy to get bogged down in it all. My advice would be not to dive too deep and fill your head with broken biscuits.
Anyway, the bottom line on all this is to forget whatever your old PM was telling you, re-test on the Assiomas, establish your new FTP and move forward.
Remember FTP is just your personal value, it’s not a measuring stick to compare your ability vs other cyclists.