 If you have read the paragraph first and then you are reading the question, then it's really easy to know that it's not given in the paragraph, so you should mark it not given. But when something is there and you can correct that sentence, that means it's a false sentence, so you should mark it as false. There was one very confusing question in my question paper was, they have parking lots on basement as well as on first floor. And the question was, is it true that parking lot is on two floors? In USA, they never considered basement as a floor, for somebody marked it as false. But it was true because considering that it is on basement and one is on first floor, so there are two floors. That's a really important point to not take any prior knowledge about the topic into the reading test. Just go on what the text says.