There's always at least a ±5% rpm difference in any comparison of fans, even with the same source. Junky fans can have as much as ±10% difference.
When you are gaming, those fans are spinning at high rpm, say 1500-3000, and with a 5% difference both ways with the center/outside fan, that's a 10% overall, so speeds could possibly be 150-300 rpm different. More likely it's a @5% total difference, but that's still 75-150rpm difference. That'll be noticeable under a light that's acting like a vehicle timing light.