One way to think about it is this: alphafold gives you the rough shape of the skeleton, but a protein, just a like a human, has a lot more going on than just the skeleton.
Also, of course, it's not just the atoms in the protein itself: add in water, ions, small molecules, other proteins, etc., and you have a real mess. A small molecule (say, a drug), can displace a water molecule, which has a cost, which causes something else to move a bit to compensate, stretching a bond, and so on and so forth, sometimes resulting in dramatic differences in the bound conformation of a protein. It's not uncommon -- it's essentially the norm. And it's not like there's just one conformation. The whole thing is really like a wiggly, jiggly, metastable mass, and the introduction of a slight perturbation is enough to send the blob spiraling into a new metastable state.
When you start to ask questions about how a protein moves over time, it becomes a much more complicated problem than predicting the overall conformation. Which is itself quite hard.