There are items used in construction that have size names that are not what the actual sizes measure.  For example, lumber sizes measure somewhat smaller than the nominal size

For example, a 2 x 4 actually measures 1 1/2″ x 3 1/2″

Pipes used in plumbing are another mystery.  For example, a 1/2″ pipe actually measures larger than the nominal size.  A 1/2″ pipe is actually nealy 7/8″ in diameter.

Included in this website are charts that help with what the actual sizes of these elements actually are.