Getting it right is definitely the exception rather than the norm. It's a silly cycle, wherein an author's knowledge of how to ride a horse seems to comes from reading other fantasy books, which they assume is describing it correctly. Thing is, even if you're writing a Fantasy novel, you should still research things. Setting it in another world doesn't suddenly change things like how to ride a horse, because the horses haven't changed and neither has human physiology. If they can't even ride a horse properly (especially those characters described as being skilled), I lose trust in the probability of other things.
So let's straighten this out: you do not ride a horse with your knees. It's not even possible.
Let's look at Harry. He has perfect form:
See his knees? See the saddle? See how thick it is? How on earth can you squeeze a horse with your knees? You can squeeze the saddle - it's instinct to grip with your knees, but I'll tell you from personal experience, you will strip the skin off the insides of your knees and they will bleed bleed bleed and the horse won't even notice.
That's right. The horse can't feel it.
Let's try another.
Great close-up shot. The saddle protudes where the knees are; it's designed to keep the knees away from the horse's body, while it's the CALF that grips the side of the horse through a much thinner piece of leather. Heels down, you use your heels by light kicks to get the horse to start moving, while you use your calves to direct the horse. It's all very subtle. Your knees do absolutely nothing. That's right, nothing.
One last image:
Just to reiterate: you can see that their knees aren't touching the horse. Even if you were to ride bare-back, you should always use your calves, never your knees, to control the horse. You spread your knees and try to keep a bit of air between them and the saddle, providing balance, that's all. Look at Harry's picture again. The knees are slightly turned out, pointing outwards. It's physically impossible to do much with them, and it wouldn't help any if you tried. The horse isn't sensitive, high up there. It's sensitive where your heels are, and your calves. It takes practice, but you send messages to the horse through the subtle pressing and squeezing of your calves, not your thighs or knees.
If the authors can't get it right, their editors should know better.
While we're on the subject, I just have to include a mention to that classic ballad, The Man From Snowy River, where the title of this post comes from - Clancy of the Overflow would wince in sympathy I'm sure.