It's an amazingly complicated difference that has to do with correlations between luck%; average flag times, table size, speed of teal sitting, number of animated avatars at the table and stuff.... I couldn't explain all, probably only skrumgaer knows all the details about this mathematical mistery

But it pretty much comes down to this:

minimum score is simply the minimum number of points you need to sit at that table

buy in number is roughly the maximum number of points you could lose if you take 7th with absolutely no dominance at all (get reduced to 1 land and stay in for like 25 turns). If you flag in time, you will never lose your entire buy in at 500 tables or higher. At 0 and 100 tables, you do lose your total buy in as 7th