They have used Samsung on occassion as well.
In some cases, Micron does not manufacture or have enough stock in certain types of chips and use other manufacturers to supply it.
This was the case with 512MB DDR modules. Micron did not produce them at one time and used Samsung (and possibly Infineon) to supply them. They may still do this and there is even a release statement on their site explaning that their memory may not always have Micron chipsets.