Yeah that nominal sizing is a total trap. Messed up a whole batch of cabinet face frames before I caught on. You just have to remember everything gets planed down from its rough size.
Man that one got me good on my first shed build. Cut everything for a 2x4 frame and nothing fit. Had to go back and learn the hard way that a 2x4 is actually one and a half by three and a half. Same deal with plywood, a 4x8 sheet is never a full inch thick. It's just one of those weird industry things you have to know.