You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
I found a few Master chests in Act 2 too. I don't think it actually has to do with the act, I think the difficulty is automatically generated by level, so if you cheat your way to level 20 in act 1 you'd probably come across Master chests, while if you barrel through the content and reach act 3 at level 12 you'd probably not see anything above a Complex. So basically, if you max out all your XP potential, you might get to the point where a few of the remaining Act 2 chests are Master level.