I maintain that this largely indicates strength of light vehicles, which is a huge component of high level play.
Note the winrates follow the strength of light vehicles:
1. Sov : T-70
2. USF : m20->stuart / AA HT
3. luchs / puma
4. 222
5. aec (lol)
USF had a higher win rate than Soviets. Why would you rate Stuart> Puma/Luchs? Why would you rate 222> AEC? Makes little sense to me.
The reason why Sov/USF was so strong is more the crazy Soviet late game and general power level of USF across all units.