How accurate is nub theory?

I went to find out my babies gender, I actually went twice to one place but in two different locations (so same company but different people running it) and they couldn't get a potty shot but told me they're certain it's a girl from the nub theory. I can't get my head around that being as accurate? Any input please? Thank you! I was 15 weeks exactly for the first and 15 weeks and 4 days for the second one as I wasn't happy about the first but the second said the same..