Something strange happened with my category on Zwift. According to my profile I was cat B. Saturday (30 december) I did an effort in the climbing portal (Old La Honda). Before that my zFTP was 257, zMAP 351. After “Old La Honda” I got a message that Zwift detected an FTP increase, and I went from cat B to cat A. zFTP 306, zMAP 351. Today I went all in on the Alp Du Zwift, I shattered my PB and got another message that my FTP was increased from 285W to 290W. My 20 , 30 and 40 minutes peak power are increased. But my zFTP went down from 306 to 302 … And now I’m cat B again… Not that I mind, but I don’t get it that I went from B to A and back in 2 days…
Any increase in weight between the old PRs and new?
Not really, and sure not the last days…
if you look at your zwift[dot]com profile, and look at the datestamps used for your 1,3,5,12 minute power, are these from your most recent AdZ ride?
As you can see it was only 20,30 and 40 min that has changed because of the AdZ climb of today.
This next screenshot is the one after climbing La Honda when I became cat A
IMO your figures all make sense.
When you did 17 minute full out La Honda hill climb your efforts stopped at around 17 minutes and your 12 and 15 minute power (shown above) were both impressive and indicating a gradual shallowing of your power curve.
Your 20 min power of 309 and especially your 30 min power of 274 will have looked too low compared to your 12 & 15 min power curve. So calculation of zFTP would have been based on what system considered your best max efforts 12 & 15 and not 20 and certainly not 30 and 40 mins. This would likely result in a power curve which would shallow/level off from 15 mins and produce too high a zFTP. [ this whole CE power curve system was, IMO, introduced exactly to counter this 17 minute effort and sudden drop off of power to remain under old 20 minute ZP category system ]
When you then put in a max effort and PB up ADZ your 20-40 min efforts now looked like max efforts and would have been used in your new zFTP calculation and would have lowered your zFTP.
In other words it thinks your recent 315 20 min effort was more reliable than your 309 effort. Your 315 reliable effort probably produces a zFTP of 302.
Whereas your 309 was probably converted to around 320 (from your 12 and 15 min powers) and produced a 306 zFTP.
Daft as that may seem we have always been told power curve works best on MAX efforts and is not as accurate when there are sub max efforts.
That’s my take on it anyway.
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Thank you Ian for the explanation. It makes sense to me now. I just found it strange that a good attempt would lower my category. Happy new year!
Something funny happened this morning. I entered the Stage 1 - Epic Race and did a good effort that placed me 1 at the finish. The only changed in my peak power was my 12 min power that went from 331 W to 337 W, that increased my zFTP from 302 to 311 and that put me once again in cat A. So that was a good test of the explanation you did 
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