Maxis, they can't book you in for an induction if you don't want one. You just say no. You don't even need to not turn up and waste time/resources.
Babies develop pretty uniformly up to 12 weeks which is why your dating scan is around then. The baby is big enough to measure but small enough that genetic propensity to be bigger/smaller than average hasn't really kicked in yet.
There are lots of things which can make dates change once you're scanned. Obviously, they can only be accurate to around 5 days either way so you have that margin for error. Mostly because babies don't cooperate . So if you're trying to measure crown to rump and they're curled up in a ball or flipping around or generally being awkward, you have to accept that you might be out slightly.
And most women don't know when they ovulate. OPKs don't show when you ovulate so can't be relied on. Sperm can live for over 5 days inside a fertile woman so even knowing when you had sex isn't a guarantee. But say you do know exactly when you ovulate, the time taken for the egg to travel down the fallopian tube and implant can vary by many days.
If you take all of these different confusing factors for predicting dates, it's not surprising there's some moving about of dates. It's bloody amazing they're as accurate as they are. Maxis, for example, you calculated that you were due on the 9th, scan said 8th. And it only moved a few days at the later scan when dating is much, much less accurate. That on its own is remarkable isn't it? That they can see so much and so accurately? I tend to think it's nothing to do with doctors being Gods and arrogant. They have to arrange care. Women do need monitoring and at specific times if they want things like certain tests or have specific needs and scans are absolutely the best way we have right now to do that. The alternative is that you base care on what a patient says and while some women may know categorically, there are myriad reasons why a woman might seize the opportunity to be less than truthful about the time of conception. I acknowledge completely though that no medical professional has the right to tell a woman she is wrong, to undermine her or to force medical care on them that does not fit with what they know to be true.
I do see the problem with them pushing induction too soon in particular. Or sweeps. But you can say no. A due date is arbitrary anyway. I wish they'd move to acknowledging that a normal pregnancy is 37-42 weeks and there's no 'overdue' or 'early' within that. Only normal. It'd save a lot of piddling about and pressurising heavily pregnant woman or making them feel like a watched pot.