Well, Monty doesn't know (or care about) your choice. He's 'constrained' by the fact that his 'probability' is 100%: he knows where the goats and car are.
If a has the car and b and c have goats, and you pick b, he opens c. And if you pick c, he opens b...
The other 1/3 of the time, you're right, he doesn't care, because he can choose any of the remaining doors at random.