85% sure it's a girl. Worried

My ultrasound tech said she's 85% sure my baby is a girl at my 20 week scan. Obviously I got all exited and went out and spent a fortune on girl clothes. Now I feel a bit foolish for buying the clothes as its only 85% chance. I'm a bit confused, how can she be 85% sure? Surely it's either boy, girl or 'not sure' My question is,.. is there a good/high chance she will In fact turn out to be a girl or should I just take the clothes back and buy neutral to be safe. Btw I don't have anymore scans and can't afford a private ultrasound.