The reason she said it won't change from now on out is because after a while some babies grow faster and some grow slower than average, but in early pregnancy estimated due dates are more likely to be correct. So they stick with the earlier ultrasound measurements when projecting due dates.
The difference between one day and the next, when it comes to due dates, is actually no difference at all. I don't know why she said "we are going to say it is the 22nd," but it actually doesn't matter. She didn't change it by weeks, she changed it by one day. You could go into labor on the 21st and deliver on the 22nd. Or you could go into labor on the 18th, or the 24th. The baby is not in your uterus reading a calendar and refusing to come until the doctor said it was OK.