First in "fitness age" shows "Increase Vigorous Minutes" a 6 week average but when you click through it shows the same value for a 4 week average.
The figure is neither the 4 week or 6 week average if you add up the the vigorous minutes (for 4 weeks or 6 weeks) that can be seen in the "Intensity minutes" section of the interface.
Also in "fitness age" shows "increase vigorous days" a 6 week average but when you click through it shows the same value for a 4 week average I have not checked these values but it's not possible to get 2.1 when dividing a whole number of days by 4. It seems likely this is really a 6 week average as 1/6 is .167 so 2.1 is possible.