I believe it has to do with the size of the wheels. Dunlops on the 16" (stock SR5), Michelin on the 17" (option SR5 and SE). Every SR5 I've seen in our fleet (some 16" some 17") is equipped that way. My 09 TE came with the Dunlops (no more). I don't think that Michelin makes a 265/70-16, ergo the Dunlops. Just my .02 worth.
Bill W
The stock tires on my 03 Limited were Dueler H/T's, so not quite sure how they chose which tire went on which T4R.