The way I see things is that the nub is made up of 2 parts. The bottom line-the flat line and the top line- your angled line. Theres a baby2see diagram somewhere which shows photos of the early changes, I'll add the link if I can find it.
In alot of u/s pictures baby can be at just the wrong angle as to not show the angled line so only leaving the bottom girlish looking line. If your pic isn't leg, (which it may well be I have seen stranger things!) then your baby was in the perfect position to show the full boy nub. Often we are only left with a little dot on top or a bit of a pale shadowas a hint. If the full nub isnt showing then sometimes you can guess off of the length of the nub. Gestation can alter it alot, the closer to 13 weeks you are, the longer the main white flat line on a girl looks so a shorter nub at 13 weeks can sometimes mean a hidden boy nub.
Forks can be confusing imo and so can angles, imo it comes down to the detail. I'll post a pic below of one of my girls, it had all boy guesses on IG when I first posted it, but it all cam down to the detail! Which wasnt there!
Just noticed the baby2see links at the top!