VO2Max comparison to other devices

I recently switched from a Fitbit Charge to Apple Watch 7 and the VO2Max went from 42-45 range to 21-22 range on Apple Watch. I tried comparing them for about 2 weeks and the results kept the same discrepancy. I also saw others asking the same problem. The set up looks correct to me, like many others who are having the same problem also replied. Was there any other responses beside check your set up? Or have anyone compared with other devices and checked out ok?