"how did I screw up?"

I don't think you did.  There's an important "technicality" you're not taking into account.

When you look at the media pool, there are seven numbered bars.  Each bar represents a variation (different key, etc.) for most (but not all) of the soundpool objects.  So, what you're missing is that there are (usually) seven versions of the same item in the media pool.  3500 divided by 559 equals 6.26, which is just short of seven, which would be about right.