Sounds like a bug to me, but I'd need a lot more information, if you receive the Expansion early, we would not be able to catch that on the backend, because we would look and see that the Expansion was added to the account already. As long as the number of Expansions added to your city (placed or not) matches
this chart, you're receiving what you should.
No one complains that they received an Expansion early, so while we get a handful of "Where is my Expansion?" tickets, they're tricky to track.