I have a character in Diablo 3 and am able to create games with the options of starting from a lot of quests. However, when I try to join a public game, I notice that my options are severely limited. I am wondering why that is?

For example, I am able to create a public game with a quest from Act 4. However, when I try to join a public game in act 4, that option is unavailable.

I am wondering how does Blizzard decide when you can join/create a game with certain quest.