Uy, confusing talaga yan! Pero based sa what I've heard from mums around, ang first trimester ultrasound (around 8-12 weeks) ang pinaka-accurate para sa EDD, that's when the baby's size measurements are most reliable. After that, the further along you are, the mas may room for error sa dating kasi lahat ng babies develop differently, kaya bawat OB might get slightly different readings.
So kung ang first check-up mo (1 month) was when you got the Dec. 25 date, that's probably your most solid one to go with, pero honestly, the difference between Dec. 12 to Dec. 25 is pretty normal variation lang. My neighbour swore by double-checking with her OB kung which scan they based the EDD on, kaya maybe ask your current OB kung anong dating ang most reliable based sa scans nyo.
Either way, best to chat with your OB-Gyne about which one they're confident with, they know your case best, and babies rarely follow the exact date anyway! Good luck, mama!