It is simply the natural oxygen carrying capacity of their blood
Pro riders have top percentile scores in this
If you or I trained as hard as humanly possible we could increase our vo2 score by 15%. So can the pro riders or anyone.
The thing is that the average score for a normal cyclist is 50
Lance and Bettini have scores of 90+
Say I have a score of 60 for my vo2. Wow, I am good! Then I train as hard as possible, add on 15%....the score is now 69%
The average Pro gets off the sofa and kicks my arse because his score is 70 without training