When light reaches matter it can be reflected, refracted, absorbed, or scattered. Light bouncing back off a surface is reflected light. Shiny surfaces commonly reflect light. When measuring a shiny pin gauge, some of the light is reflected making the pin measure small.

To adjust for this and accurately measure a pin gauge, you need to close down the substage aperture so that the light hole is the same size or smaller than the pin gauge you are measuring.

Please see the picture below.