The whitening at the base of the letter is stress marks in the rubber.
I have the exact same thing. It's the direct result of me rubbing a curb (I realized it when I did it and got out and looked). It was just enough that the raised letters rubbed, but not the wall of the tire. Notice that it's only this way on the top side of the letters (the outside of the tire). If you look close, you can also see some light scuffing on the sidewall in your photos.