i read something the other day about this. this guys on PR was basically saying he believes it has something to do with using different precursors to synth it. he said back in the 90's they used safrole to synth it. nowa days they use more PMK to synth it. he said it had to do with large amounts of safrole being siezed so the cooks had to come up with a different precursor to synth it. basically he was saying there is a differnce in the MDMA made with safrole compared to being made with PMK. not sure if this theory holds water but it sounded good when i was reading it last night lol.
That sounds like a very good theory - after all safrole is like importing an illegal drug almost, it makes sense.
http://www.bbc.co.uk/newsbeat/10353130