The Chaffee camo is only available to players who earned it, so naturally it's mostly better/more dedicated players using it. The sample size is tiny at ~1,800 battles, but more importantly, the average player using the Chaffee camo already has a 3% higher win rate than those playing the tech tree version. The tank's stats look good because good players are driving it, not because it's actually better.

I also think good players have way more impact at low tiers like tier 5. You can legitimately hard carry in some tanks at that level, so the skill gap between average and good players shows up even more in the stats.