Your local miner is your raw speed.  It includes rejects and hardware errors.  The pool side is your effective speed.  BTC Guild only counts accepted shares (some pools include rejects) for measuring your speed.  You will *always* see a variance on any pool.  BTC Guild's average gets more accurate the longer you're running (up to 1 hour), but even at an hour of information your estimated speed will always be 5-10% above or below.  For ASICs, it will tend to be on the lower end of that range due to hardware errors, which normally account for 1-3% of the difference.