Why do some people think Truvada PrEP is 100% safe?

I see a lot of people in online profiles, saying they are using PrEP medication and are HIV Neg & are therefore completely free to safely have sex without condoms.Yet, all the time PrEP is mentioned in a professional heath/medical context it is only ever presented at 99% effective.What is the story here ? If 1 out of a 100 times on PrEP you had sex with someone HIV Positive (especially if they didn't know their status and weren't on medication) and could could contract HIV - surely you're not that safe ?