Not sure that I follow what is off, but you said that you checked it with your speed square; are you certain that your speed square is right? I would not just assume that it is. Sure, it might cut square ends, but, as you said, any error is magnified over a longer run, so it could be off a bit and not show up when crosscutting a 2x4 or 2x6. Not sure that I can explain it well, so hopefully this makes sense, but one way to start our is to use a flat surface with at least one straight edge, a piece of plywood is probably as good a place to start as any, make a mark at 90 degrees, flip the square over, and see if it matches. Depending on where that takes you, I definitely agree with Mike's question about what good is a measuring tool if it's not accurate, but many time you get what you pay for, so, for full time cabinet layout, you probably want to start at Starret grade and go up from there, but, for cutting drywall, how much do you want to spend to ensure a 1/32" gap along a stairwell or in an irregular corner versus a 3/32" gap.