Its normally down to the process in which they are made / or the place they are stored.
MDMA should have no real smell! A lot of people simply think that a nice aniseed smell = good pill. That's really not the case and should never be used as an indicator of a good pill.
I could take an over the counter tablet - and within seconds change the taste / smell to indicate MDMA.
I know a lot of people will want to post that x pill smells like aniseed and they are great and I'm not for one second saying they don't or they are fake.