I would bet that rumor is false, or that there was some sort of extreme extenuating circumstances that we're not privy to.
But, as I say, I could be wrong - I'm not an American so I haven't seen these forms too much :-)
If you get down to just numbers, the 'standard' way of valuing any enterprise is to calculate the current value of their future cashflow. This is why (many) V.C.s ask you for a revenue projection. It is debatable whether this is the right way of doing things; in the case of most startups it may be the wrong way, but if id Software was making, say, only 10M/year at the point they got bought, and left alone, if their cashflow would have stayed the same or stagnated, 105 million would be 10.5x their yearly profits which is decent.