Doesn't make sense in a push/pull config to have one fan running at a different speed than the other.
It does have sense, but I'm not good enough in english to explain.
1/ Cold air and warm air don't use same space because of it's thermal expansion.
2/ the S3 is not full close, air can flow outside by many ways, not only by the pull fan.
Because of those two points, input air flow may be different from output air flow.