Due date calculators are generally considered accurate within 7 days. Typically, most practitioners don’t adjust due dates in either direction if the discrepancy between the two possible dates is less than 7 days. And that’s true even of early ultrasounds.
There are generally two ways of measuring a pregnancy, LMP (last menstrual period) and gestational. Most due date calculators go by LMP, which calculates a pregnancy at 40 weeks or 10 lunar (4 week) months.
LMP would mean that ovulation took place in your 2nd week of pregnancy (day 14). Gestational starts from the date of ovulation resulting in a 38 week pregnancy. So if you know the date of ovulation you can calculate where you are by either LMP or gestational.
The ultrasound technology may give you an odd date because they may not use either of these; they sometimes take the rate of development and change the date based on where on their scale the baby is. The Doctor may even move the due date based on this, even if you know absolutely when the baby was conceived!
Babies only have so much possible size variability throughout pregnancy and so most will be of a certain size at a certain point in the pregnancy. The later in the pregnancy you are the more individual variability among babies there is and so the less accurate the size and date estimate will be. However, for example, almost all babies will be 2 inches at 13 weeks (LMP) and each little bit bigger than that they are, an extra day would get tacked on to an ultrasound estimate.
The solution is to calculate the due date and then, unless there are complications be ready early but figure on going late. Many people have their hearts set on their due date and are so ready on their due date that when it goes by and nothing.
They get pretty upset.