I ordered my stuff from 1stSubaruparts/Auburn Subaru. They specified the correct belt, part #809218400 (Gates 5PK877), the same I'm taking off. I received part #809218340 (Gates 5PK874), and they're not sure why. Apparently '340 is listed as superceding '400, and that's why I got '340. The Gates website shows I need an 880mm belt, the 877mm seems to fit well, but the 874mm is too short. Why would Subaru change the belt to something shorter? 1stSubaru is sending a longer belt, so they're making the effort and I appreciate that. I expect to be doing many more belt changes on this car, hopefully I won't have to go through this each time.

Who knows...they may have just got things mixed up in the system. As long as they're making good on it, I wouldn't worry about it.