I had two roommates try to make a structurally stable stool / box out of lumber. 3 hours later I returned to find both of them pissed off & a "box" without a square corner in sight.

Days later, I offered in a conversation with them how odd it is that dimensional lumber doesn't measure the named dimensions.

They looked at me in shock and said "Maybe that's why nothing was lining up."

That's "X does Y, because how hard can it be?" in a nutshell. But the broader takeaway is to test & validate your own assumptions -- they had a measuring tape the entire time.