Caldwell Chronograph readings?

Hey all,
I just purchased a Caldwell Ballistic Precision Chronograph, shooting a Bowtech Realm at 28.5 inch/70.5lb draw. The chronograph at the bow shop rated my 475 grain fmj's at 260 fps and my 400 grain black eagles at 279 fps, although I have only used it a handful of times. The new one I got consistently reads high, saying my fmj's shoot between 265-271 and my black eagles shooting 287-291. Any idea as to why this is, and which reading I should trust? Anybody else have this chronograph?

 

Does it really matter? Its 10 fps off one way or another. If its consistent dont worry about it.

 

I am not worried about it, however I shoot the bow every day and like to know as much about it as I can. Just curious. Simply asking if anybody else has encountered something similar.

 

Very common for chronos to be calibrated slightly different.