Sneak Peek testing

So the sneakpeek site says you can start testing for the gender at 8 weeks due to the x and y chromosomes developing at that time. I took my test at 8 weeks 2 days and got the girl results!💕 Well I went to the doctor a couple days later for my first ultrasound and my nurse said I was measuring 7 weeks 5 days. I have felt since the very beginning that it’s a boy. Just a deep gut feeling and tons of dreams of a boy!💙 I’m happy with either, but could the test have been wrong since I was measuring a little behind? I know that I’ll truly have to just wait until my gender ultrasound, but any feedback would be nice! Thank you!❤️