Good question, original publishers (the magazines, in this case) set the terms for how long an exclusivity period they want to purchase for first print rights. It's usually a year, but many magazine contracts have a carve-out for "year's best" anthologies like mine.
I deal directly with the authors, not the original publishers. I pay a fixed rate for reprint rights, which is a one-time deal. This is mostly because pro rata royalties on an anthology are a pain to calculate (I've done this before), but also because it's unlikely for me to make back the money I spend on anthology creation, so if there were ever royalties they would be tiny in any case.
There's not much money in short fiction -- many authors tend to use it as a stepping stone to novels, or just as a hobby.
