Oh gosh something I can actually explain.
One of my first jobs as a teenager was working in a s*ucker factory. We made Kiddi Pops - check the back of the bag; they are made in Dalton, OH.
Candy is made in "runs" meaning for the first run of the day we may put in rootbeer tack (tack is the term used for the process that results after syrup, sugar and flavoring is melted in a pressure cooker, and dumped into a machine) and the way candy is kept decent is that behind the rootbeer flavored tack they would made butterscotch. Something that would NOT be nasty should a few lollis get through with BOTH flavors. Fascinating really. One of the "easy" jobs in that factory was catching all the lollis that came on a mix run - and throwing them out, to be recycled and boiled (paper on, sticks in as they float to the top) and then skimmed to pour the "syrup" back into a boiler, mixed and used again. THIS is why you will NEVER see me eating any s*uckers. Once you see the stuff that is swept up, and dumped into a vat, then boiled - BLECK!
So a lot of time you will find - black cherry with a taste of raspberry. Cherry behind that, and on and on lemon after lime....etc.
I would imagine that the same happens with M&M's.
Just my thought - but actually if I found M&M's like that - I would call Mars Co. and tell them. They will take your name, address, and you will more than likely be asked for the code etc, and given /sent coupons or samples that are perfect.