FTP as Calculated by Garmin

I have a Smart Trainer and also a couple of 4iii Power Meters (left side only)

When I bought a Garmin 1030 last year it forced me to use the Garmin Android App (before I just plugged my 800 into my laptop and let it sync to Strava

With the Garmin App you get all sorts of other data that I started to look at - one of which is FTP. I am not sure how it calculates it given I have never gone out to do a FTP test either on the turbo or on the ride - hence I don't think I really believe it. However it gives me a relative benchmark from ride to ride (when measuring power).

I find that it's actually quite demotivating how it seemingly varies relatively easily

Here's a screeenshot of my last 6 months:

My "Garmin FTP" rises during the summer, then drops away over the winter - I peaked at 216W in 2019 before it bled away. I managed to push it back up from about 195W in December 2019 up to 210W in late February by putting some big efforts in on the turbo when riding using the RGT trainer application. I found that the "hilly" Pienza, Paterberg and Ca de Formentor routes suited me to push the power. The flat routes - 8 Bar, Canary Wharf and Borrego Springs don't quite do it for me.

I don't believe the real value and if I did a proper test I am certain it would be lower that this - but it gives a benckmark to compare myself to and equate how I felt on a ride vs. what the numbers say

It also gives a VO2 Max value - I believe this does need a proper test so how this is achieved I don't know. I guess I need to read and see if they publish information in line. However whilst the FTP number is not that great, the VO2 number rates me as between 49-50, which for my age (let's just say it's 50+) rates me as having a Fitness Age of 20 - the top 5% for my age and gender - I'll take that.