Had a 38.8 reading running 3.29 km, for 22 mins at 6:49 / km pace . Avg heartbeat is 167

4 days later , Dropped to 36.8 reading running 3.57km, for 25 mins at 6:33 / km pace. Avg heartbeat is 168

I feel I had improved in the 2nd run , yet vo2 max dropped to below average for my age at above 40 years old .

Anyone knows how this is calculated ?