16 inches on center across 12 feet is actually 10 joists, not 9, so the math gave you that extra one for a reason. And yeah, measure from the same end every time, that half inch drift is real. On a freestanding floor like that I'd frame it like a box, rim first, then run your inner joists off the rim with hangers.

Did the interviewer actually let you keep going or did that kill your chances?

Eh, sounds more like you let one dude get in your head.