Fitness Age (Fenix 6x)

Currently using a Fenix 6x (previously had a 5x), and noticed something unusual about the fitness age.  Over the last year, my fitness age has not been very good, with a VO2 max of 36 at the moment the Garmin app on my phone says "Your fitness age is 57.  Thats the bottom 30% for your age and gender".  This has reduced from a fitness age of 66 over the last 12 months as my VO2 max has improved (I am actually 41 years old).  This is fine, however, today (and I have not done this previously, I normally just use the garmin app), I logged onto garmin connect website from my PC and under "reports" I click vo2max which shows me the pretty colourful pie chart thingy with the correct VO2 max on there but it is displaying a fitness age of 45, which is much better obviously.  But over the last 12 months I have been working down from fitness age 66 to 57 - which one is correct, the web site or the mobile app?  My device has synced correctly and is showing the same data as my phone and watch, so not sure whats going on.

Anyone have a similar issue?