An extensive post on the Natural Selection Forums offers details on a bug they've been battling that turns out to be present in the Half-Life source, rather than their mod. The problem seems connected to client prediction where the final player on a full server (i.e., player 10 on a 10-player server) is difficult to hit because the visible character on screen is not in the same location as his hitbox. They have determined that this problem is present in other (perhaps all) Half-Life mods, though it is not clear if it appears in unmodified multiplayer games. The issue also seems more evident the faster the bugged player is moving, which may be why it was more obvious in Natural Selection games.