I see a few possible explanations: (1) A woman who sexts could have multiple sexting partners. In the extreme, you could have every man in the world sext with one woman, making the male sexting prevalence 100% and the female near 0. (2) While we defined sexting as "sending or receiving", some respondents may interpret the question as primarily about sending. There could be a gender bias in the sending vs receiving of sexts. (3) As you point out, the data is reported behavior and not observed behavior. Reported behavior often is a good proxy for observed behavior, but it is not perfect. And there are known to be effects where certain demographics answer questions dishonestly for conscious or unconscious reasons. Perhaps women are less willing to admit to sexting behavior.

Even assuming all of the participants are being truthful, you'll have some problems with definitions not being shared by the participants. The classic example is "Does oral sex count?" Men, for whom a large number of partners is (sometimes) culturally good, may be inclined to count it. Women, for whom a large number of partners is (sometimes) culturally bad, might not count it.

With respect to sexting, imagine a woman sends a man a picture of herself in a low-cut dress. The man might say, "she was totally sexting me", and believe it, the woman might say, "I was fully dressed in an outfit seen by hundreds of people at a party, how can that be sexting?", and believe it.

How would you be able to tell?