Probably it's a mix-up from the BCIS side.
When a case is denied, the petitioner either can go for appeals or can request a "motion to re-open." In your case, it seems that you did not submit any motion to re-open (I am not sure whether you had any similar denial in long past for any other NIW petition if you ever filed and denied) and still they re-opened an approved case! I don't recollect any similar instance in last 3.5 years of passive observation.
Does the letter mention about receipt of "your" request for motion to re-open?
Best wishes,
Jigesh
Disclaimer: Personal non-legal point of view only.