Mathamatical Rounding error? Or is something else at play. (edit: solved not a bug)

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 2’3" (27inches) - 3.5 inches -3.5 inches = 1’8" (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.

Most likely, snap length is enabled in the Model Info settings, which not only affects drawing, but also moving. However, it does not ensure that you draw on an absolute β€˜grid’ when starting a line or rectangle. It just uses a pixel on the screen and calculates the coordinates.

@egodsey thank you for point this out. I was unaware that I could do this.
In my case you were right. the 2x4 (1.5"x3.5") was actually 1.5"x 3 31/64ths AKA my precision was set to low and I was not able to see as to how small my actual 2x4 was. I have length snapping set to on. I will change this to 0.5 inches to prevent this in the future.

Thank you @MikeWayzovski as well for the info.

Again, many thanks.

Better turn length snapping off. It is a totally useless feature that no one in their senses should use.