Board Thread:Plants vs. Zombies 2: It's About Time/@comment-5447380-20140208142017/@comment-5565348-20140218210115

Because the 25th levels of Pirate Seas and Wild West are technically there, just not accessible(presumably may be unfinished). And the key you still have is a World Key, used to unlock other worlds. (apparently moving from a pre-1.7 to a post-1.7 has your stars turn into completed levels, unlocked worlds are instantly unlocked, and I believe that you are given a World Key(there may be some extra conditions, I believe we both got all stars).)