My VO2 max is different from my watch's
Why Not shows one VO2 max at a time, from the best source it has, and the line under the number says which. In order: a VO2 max your watch measured and saved to Apple Health (an Apple Watch's Cardio Fitness), the number you entered from your watch's app, then Why Not's own estimate from your heart rate and pace.
The estimate reads your outdoor runs that hold at least 10 steady minutes after the first 5, at 70 percent of your max heart rate or more. It sets how fast you ran against how hard your heart worked, using your resting heart rate from Apple Health, and it needs 3 such runs in 30 days.
Race fitness is a different number, and it is never shown as VO2 max. It is the number your training paces come from: a race or time trial in the last six months, the hard reps of a session run at their target pace, or the heart rate of your easy runs that qualify.
Watches differ from each other too. Each watch maker uses its own method, so one runner can read a few points apart on two watches. Every one of them is an estimate; only a lab test measures VO2 max directly.
To use your watch's number, open VO2 MAX from Home and tap Use my watch's number. Enter the number your watch's app shows and where it is from. A watch's number moves with your fitness, so after 30 days the screen asks you to update it from that app; after 90 days without an update, Why Not stops using it, says so, and shows the next source.
Why Not, Home, VO2 MAX, Use my watch's number.