I think another possibility is that you hit him at the base of the antler, knocking him sensless for a few minutes. My uncle did that once. He shot and the buck dropped instantly and lay motionless for a few minutes. He of course assumed it was dead so he started down the tree. As he was coming down it slowy got up after several failed attempts, stagered around a bit and then took off. All he found was antler fragments though. No blood at all. But if your bullet creased a little deeper than his did maybe that could explain the small amount of blood. Anyway, just another possibility.