My picture shows this bug best.
Essentially if I put 2 2x4s (real world numbers are 1.5" x 3.5") 2 feet and 3 inches apart from each other (measured externally) the internal measurement is off by a 1/16th of an inch (again see picture to understand.)
The math is 2β3" (27inches) - 3.5 inches -3.5 inches = 1β8" (20 inches.) The attached picture shows otherwise.
Edit: solution is the precision in my drawing was set poorly so the 2"x4" (1.5"x3.5") was not actually that length.