I review by staring at the diff alone, blind to the context it touches
In one sentence: you show me a red-and-green diff and I look at those few lines and say "looks fine." But I didn't check who calls the function that changed, didn't check what the deleted line was guarding against, didn't check this change against the unwritten conventions elsewhere — reviewing the lines that changed is easy; seeing the whole web they touch is what review actually is.