Eightbore,
I often use small bore gauges and have recently discovered the cause of some erroneous measurements. If it is used near the upper limit of that particular gauge, the moving wedge may be a good bit behind the contact points. If you then tighten it up too much, the contact points may deflect, resulting in a measurement larger than actual diameter of the bore. I did note you are concerned that your measurements are smaller not larger than expected, this time.
Mike