Okay long story. Possibly some tmi

So.. we have been trying to conceive another baby. On the 21st of November I had ewcm the next day nothing.

We went to the doctors at what we thought was 8ish weeks they measured the twins and they were measuring 7 weeks 5 days.

Here is the notes from my appointment.

My question is, with the due date given that would mean I convinced on the 25th? But only had fertile cm on the 21st that entire cycle!? That or are they older then they are measuring? I’m confused. I know she talks about how they measure the due date or - one week and a part where she says fetal measurement are 2 weeks behind menstral age? Does that mean they could be older and smaller? And wouldn’t this make sense if they split one egg now being two obviously wouldn’t be as big?

I appreciate any knowledge as I’m a bit confused! Thanks much!