Did you get an email acknowledging your Prestige status and did you select Prestige benefits in mid-December? If you passed the Elite mark in December, your new status would only be acknowledged by email in January and you'd be invited to select benefits in the past two weeks. So if you responded to the earlier Prestige invitation to select, that's why you got that package. AC should have locked you out until the January sweep.
Another stupid move by AC since the difference between P and E is 10K there are many who had achieved P in November when the first sweep was made who could well end up with E (as the OP seems to have done). In the meantime, they locked out SEs who were in the 60K-70K range expecting they'd earn 30K-40K more in one month to achieve SE again! Stupid, stupid, stupid AC IT!