Baby always measures almost 3 weeks early

At every ultrasound my baby girl measures 3 weeks bigger they say she is very healthy but they don't change my due date or anything so I was wondering if I would have her sooner I'm due July 24? Or am I just gonna have a big baby? In the first trimester I found out I was having fraternal twins and at 12 weeks there was no heart beat for twin B and ever since then twin A measured 3 weeks earlier. Could that be why twin A is 3 weeks bigger in size?