These particular numbers (or the underlying population it implies) matters because of multiplayer.

More players = better skill matches.
More players = better connection matches.
More players = quicker matching times.
More players = more playlists to choose from.

If you want to play with other people in your game, the further up the list you go, the better chance you have of that happening. Go lower down on this list, and eventually it becomes impossible (literally) to get a match unless you organise it outside the game first.

That said, these relative placements are fairly useless if you're trying to make any real point about one Halo game vs. its sequel. Or one game on the list vs. another. The data is insufficient. For example: there could be 3 players separating #1 and #2, or there could be 3,000,000.