Gerry
So what if the garage one doesn't agree with either of your others???
I'd settle for one which you feel comfortable with using. The way I look at it is that the psi will be different from standing still to driving for an hour and summer and winter may affect it as well.
Maybe I'm wrong but as long as the pressures are consistant each time I check and the same side to side I when I check then I'm OK with it.
I agree though, it is frustrating that you can't just buy 2 different guages and get the same answer from both!
Safe driving!