Average Vigorous Minutes reduce the more i exercise

My Garmin Connect switched to Last 6 weeks average when calculating fitness age and my weekly average vigorous minutes seem to be reducing the more I exercise and gain minutes. Anyone having the same issue? Interestingly, estimated sweat loss was 9 ml in a 45 minutes running activity at zones 4 and 5.

I am using Garmin D2 Air X10, which is similar to a Venu I guess.