So i'm playing MPH and the last time I played it was a few months ago. I get in a match and get the sniper and i'm barely paying attention and my shots were off and i'm getting headshots >_>....and as you know i'm not a very good sniper when people are moving
Try the Metroid Prime Hunter board...
http://nsiderforums.net/index.php?board=20.0
Anyway, that sounds to me like lag (if you were on Wifi). Your opponent appears to be off to the side, but he's really where you aimed even though on your screen you see otherwise.
It can be annoying because sometimes you'll get a headshot but it won't count since your opponent is really a little bit over. So lag can be good or bad....
One word, as ErrorCode said: Lag.