Well, one would think that it would just display 2 pages onto one like that, but...
I'm guessing it changes mathmatical calculations. Like all search engines, google is based off of matrix algebra. It performs some calculations, iand then it spits back the best fit answers and matches upon a number of things. Your changing of results per page must change the size of the initial matrix, or some variable held within, would be my guess. (This sound complicated and weird, and it is... it's also dumbed way, way down, so it's a bit inaccurate, but it's the right general idea😛