The point is that it isn't a precision fit. It called a 2 1/16 guage because it is sized to fit in a 2 1/16 hole with some clearance. They always fit pretty loose in the brackets. If the bracket hole is big enough to slide the guage in and small enough so that the guage flange covers the hole it will work fine. Stewart Warner says their guage is 2.04, VDO is 2.05 and Autometer just says theirs fits in a 2 1/16 hole. Since you are wanting a more precise fit then you did the right thing by measuring the guage.

In theory, there is no difference between theory and practice. In practice, there is.