Completely surprised at my 20wk anatomy scan

So I was having issues obtaining health insurance the first part of my pregnancy so I didn’t take the NIPT test at the time it would usually be done. I paid for an elective ultrasound at 15 wks just to make sure my baby was alive and well and did a sneek peek test… the ultrasound tech confirmed I’m having a girl and complained my baby was so low in the pelvis and printed a few blurry pics (complete waste of money)… fast forward I go to my first prenatal appointment at 20wks 2 days they send me for a ultrasound the same day… the tech starts the scan and first thing she asked “what did the elective ultrasound place tell you?” I said “she said I having a girl” the tech then says “Well there’s 2 babies in there”. I’ve never been that shocked in my life!!!! I’m having twin girls!!! I’m so excited but can’t help but feel cheated out of knowing sooner, like how did they miss a whole baby???