The user observed in T88240 only discovered to be in a Wikipedia Zero program by seeing the checkbox in the settings of the app. He didn't remember ever doing something special other than downloading "the very normal Wikipedia app".
I don't know if Wikipedia Zero was really available with his Italian phone operator: perhaps the checkbox, even if enabled, wasn't doing anything. At any rate, users seems totally unaware of what service they are using or not, which also means statistics are skewed by including people who don't need Zero rating and by not including people who would need it but are unaware of it.