It's occasionally happened to me. We rent Bollywood films and British stuff, so I think on the rarer things they just have less copies.
And perhaps on the high demand stuff (like possibly Bender's Game) maybe they don't have enough copies to meet demand, and maybe one location has to many, so they ship from there.
I don't think it'll happen often though. it's maybe been like 4 times in the four years that I've been a member.