For an equality operation, all bits are meaningful, as there are input configurations in which any bit can lead to a change in output. However, I do see your p

For an equality operation, all bits are meaningful, as there are input configurations in which any bit can lead to a change in output. However, I do see your point in that whether a bit flip in input leads to a change in output depends on the particular input. My counter to that is that it is not the specific input, but the potential inputs that matter.