The valve deposits on a DI engine are from oil vapor coming back through the PCV system and collect on the valves because no fuel is washing them off.
This is my understanding as well. I'd only add, in the case of my car, it is this & also from the EGR. At least according to all the pro engine builders & 3rd party mod companies (Cobb, CP-e, etc.). This is why on the mazdaspeed forums they recommend a PCV solution like a catch-can & also an EGR block plate, along w/ a COBB AP tuner, which can permanently delete the EGR CEL.
As far as the mention of uneven distribution on intake injection, this is true. There's been a lot of testing on this & this can be an issue on a stock intake. However there are aftermarket intakes that have very, very even distribution so if at least for some cars this is one way to get it to work for you. Though these are all ideas that will give you warranty concerns so keep that in mind anyone considering it.
