There's actually 260 probable items out of all chests, but a lot of the rewards are repeated. It may just be that the probability is based on how many times that item appears in the rewards, rather than it just being a few items with large % chances.
For instance, in I-rank chests, 26/260 of the probable items are 150g, thus making it 10% overall.
No, it's purely percentage based (though naturally, due to limitations of a PRNG, it converts a number between 0 and 32767 into a percentage to decide which item you get). Heck, the game's hardcoded such that it can't handle more than 32 data entries from the same chest. Of course, that cap's never reached since even Rank 10 chests only have 21 possible results, but it certainly couldn't handle anywhere near 260 different items (and there would be no sense in coding it that way anyways, since a table of limited items with percentage chances takes up *much* less space and is easier to handle).
No it isn't. You're misunderstanding what that list is showing. That's showing how many seconds you have to wait to abuse the RNG, and the only reason it says 260 there is because that's how many seconds that list is calculating. It could calculate as many seconds as you want if you set it high enough.