Why can't these other countries genuinely think whatever motion is under debate is just/unjust and vote accordingly? That seems like the obvious null hypothesis.
Well, if you consider that countries do try to persuade others of their righteousness, including at the UN, this is like asking:
Why does anyone assume that advertisements influence consumer behavior? The null hypothesis is that everyone just buys what they think is best for them.
The fallacy here being (1) that "best for them" is hardly a simple matter to decide in many instances, and (2) that billions (if not trillions) would be spent on advertisements [or lobbying & propaganda for the political equivalent] if it made no difference, and (3) once advertisement is internalized ("I think Pepsi is better than Coke, I really do"), you've made your reasoning circular/trivial if you stop at "everyone just buys what they think is best for them" without trying to figure out how they came up with that (Did they just taste both in a blind test? Did see any ads that maybe just told them that? Or maybe it was the packaging that had some influence? Or the price? Were they promised a rebate if they just say that? Or just given a free cookie [a.k.a. aid between countries] by one of manufacturers beforehand?)
(And yeah "what is just" in politics is not like in math.)