Cozzi asked the same thing.  The answer is no, I totally made it up.  Most of the time we "need" more than 100 tiles for rendering anyway, so the value of 100 basically means unload everything you don't need right now.  A higher number would keep tiles in memory when you, say, zoom out and then back in, which would be nice.  On the other than, having memory usage drop when you zoom out is a nice feature, too.  I don't know the right answer.