There are so many different methods out there for calculating VO2max using power, HR, weight etc that very few if any of them provide exactly the same answer.
However you don’t really need to worry, from a Zwift perspective, as having introduced it they have now decided to remove it.
This link gives you latest, I think, info on Category Enforcement:
And if you want short version this is key section:
2. VO2max
With the changes mentioned above, it didn’t make sense to keep using the VO2max value for the category determination for two reasons:
- Because VO2max is a metric that already includes weight normalization, it uses the weight currently set in the user account. It is also calculated at the same time the other power metrics (zMAP, zFTP) are. Continuing use of VO2max with these changes would cause inconsistency with the weight average that will be used for the category enforcement moving forward.
- The use of zMAP - a proxy of the user’s maximal aerobic capacity - allows us to discontinue using VO2max in the category determination.