What are you doing measuring inside?
Wall thickness (by subtraction)
And,
Checking a hardware store bushing I use to standardize shoulder measurements.
Digital caliper was coming back wit .336 but it slid easily over the neck of something measuring 0.339 with the outside jaws...
Vernier gave 0.342 (converted and confirmed with digital by measuring outside of the vernier ) which makes more sense.
Clearly the design of the jaws matters....
