I've heard and read before how square footage is arrived at. Tricky , I believe has got it right. However, I have seen many times , layouts that were very suspicious. The numbers always came up way too high(surprise, surprise). Can't recall ever seeing one that came in looking too low though.
Never ever take for granted what you see printed in black and white in as as being accurate. If your initial gut feeling conflicts with the square footage you see printed.., go with your gut and the do some careful math.
Quick little personal story:
Many many years ago we bought a side split from plans and it measured as 1510 square feet.
Some friends of ours, by sheer coincidence did the same on the same weekend but at another development by a another builder. Their side split measured out at 1595 square ft.
For months we excitedly compared plans when we got together. They loved theirs. We loved ours. A number of times a comment would be made from them that it somehow seemed odd but our house on paper seemed to appear larger in many respects. Of course we all agreed it couldn't be. The plans total square footage said so.
They moved in. We moved in. When they came to visit us, they were shocked. Our house was not smaller or similar, but obviously larger. Without trying to sound smug, I think all four of us learned a new life lesson then.