This is more "the law of averages" coming into play than any real evidence. If you flip a coin 20 times, its very probably to hit 18 heads. If you flip it 1000 times, you are really, really likely to see about 50/50.
Cards are not made by boxes, they are made by print runs. I don't know what the print run is, but I can assure you, its not a case or each case would be the same. Given that there is upwards of 50 rares in each set and not every pack that has the same rare is the same pack, if it were done by the case, that means each rare would only have about 3 possible different packs, which is not the case. I would venture to say there are easily over 1000 different possible packs in a print run, which means the case theory goes away pretty quick. The Rares (by this, I mean holos, EX, FA, etc, anything that can occupy the rare spot) are distributed across the print run as the design team sees fit. Theoretically, there is supposed to be some rhyme or reason to it, but I highly doubt it...