Certain acts carry more risk than others. Being the receptive partner also increases the risk. On the other hand, their are other variables too; STDs - partners viral load (if known to be + in the first place), which are medical aspects of the risk. Then there's the personal aspects; where a certain person is from - sexual orientation...
That is why sometimes people are told to take PEP and others are not.
Bottom line, HIV isn't a game of odds. When you have a risk, you MUST get tested, and that's the only sure way of knowing.