You've gotten a lavish, extravagant upgrade to the poshest seat in the Comfort Plus section of "Main Cabin", and you don't understand why Delta has chosen to notify you of the bonus that has been bestowed upon you?!?
Or . . . .
To, kind of, use a version of some lingo from another regular poster here:
You've gotten a lavish, extravagant upgrade
from the fare class and seat for which you were willing to pay . . . .