Because as of right now, numbers, you are telling me that Sam either waited a year to go to college/did the five year plan (um no) or that Dean has an extra year in there somewhere (more plausible) but the numbers still don't make sense because we have canon ages and birthdates.
Why am I being so OCD? Brain you fail me today.
ETA: Okay, cause I'm crazy. *waves list* I did out all the years and their ages have never been goofed on the show. It's that their ages DON'T MATCH traditional schooling years in the US. That means they have an extra year in there.