That doesn't explain why there's a specific pattern. Or why the gap is THAT large, which it shouldn't be
I'd suspect that all the lane maps (White Ball, Red Ball, Hamburg and to some extend Arnhem) being the most popular maps have quite a significant impact on the Allies/Axis global win ratio because those maps are relatively easier for the Axis with their HMG 42s and especially superheavy TDs, and because most casuals have a hard time dealing with them. Sadly top 200 3v3 and 4v4 data is such a small sample size that it's unreliable, but at least in 2v2 with a bit bigger sample size it shows that within the top 200 the Axis/Allies WR is mostly fine at 52/53 vs 49 (with the exception of UKF, but again that could be skewed due to low sample size). I'd consider anything between 47-53 to be good enough.
Also the Allied factions seem to rely more heavily on light vehicle play, as indicated by high WRs in 1v1 for USF and SOV (but not UKF who lack an AI LV), which makes them very good at 1v1, but less so at teamgames where LV play is weaker.
UKF is not doing well, but that's why they are getting a boat load of buffs to their doctrinal arsenal in the upcoming patch. Along with a heavy nerf to Jaeger Armor.
I wish we could add the Land Mattress and the Calliope to the stock rosters of UKF and USF, as I reckon that would improve their performance (mostly on the lane maps) dramatically, but sadly that's not going to happen.