In my experience, it's because they think some encounters don't "count" because of Western society's mess of clearly defining what counts as "sex", so she rounds down to look better.

Just blew him outside the bar & kept her clothes on? Didn't have sex.

They tried but he got whiskey dick and couldn't get it in? Didn't have sex.

He went down on her and she just jerked him off? Didn't have sex.

His semi-chub only got halfway in and came in 5 secs? Didn't have sex.

Got too wasted and can't remember? ... probably just hand stuff. Didn't have sex.

Only did anal? No dick her in V. Didn't have sex.

Just dry humping till they both came? Definitely didn't have sex.

Unless you're unironically taking American Pie as your sex bible...