It sounds to me like you have an issue with your scope. Are you using the same scope settings at both ranges? Do your cross hairs block out the bull so you can't hold as accurately? It may be that you can only really tell with a stationary rifle vice. By stationary I mean one that holds the rifle securely AND is fastened so that it doesn't move from the bench. Unless your load is transitioning between sonic and subsonic between 100 and 200 yds (most unlikely) nothing else makes sense to me.