The reason is, my blades report 12.5-13GHS at slush and it goes down to 11-11.5Ghs when mining on bitminter :S
Stats here at the mint are more like a guess - at least not accurate.
When you completed at least one shift, the "your ghps" value on the live stats page should show a better value.
I couldn t even know if my miners were working correctly at slush on its front end. Most of the time i was seeing them not hashing at all or only some of them sometimes even though they were on and getting shares. I contacted slush about it but never got an answer. Doc haribo s always replied to any of my emails sent to him. My stats are way better here at bitminter than at slush. Moreover now we have black lotus protection from DDoS attack
Well it s a matter of point of view, habits and trust regarding the pool i guess... :-)