I don’t know anything about the book in question, but I assumed it was because of the way the hook at the end of tape measures has a little play to it. If it’s a poorly made tape measure that doesn’t account for this properly, you get slightly different measurements by hooking around something versus pushing it against a surface.
(See point #5: https://www.ustape.com/hidden-features-of-measuring-tape/)
If it’s a poorly made tape measure to begin with, doesn’t really matter. Could easily end up with 1/12" off.
Oh right, fractions of an inch are usually expressed in fractions of powers of two. SI humour went right over my head because I’m used to seeing weird fractions involved in those measurements.
Feet are divided by 12 but inches are divided by 8. And a yard is a foot times 3.
It makes way more sense when you use it, but trying to explain it does make it sound crazy.
