There are a lot of places in the US that sell ``pints'' of beer in ``cheaters'', which are glasses that appear to be 20oz glasses, but which have over-thick walls and bottoms, so that they contain well less than 20oz. Are you sure that you got 20oz?

Of course, that wouldn't explain why you got a differently sized glass than your colleagues. Was your beer on tap and theirs from bottles or something? Maybe you got a 16oz glass and they got a 12oz glass (bottles in the US are 12oz, IIRC)?