why does my boyfriend appear to be online on facebook when he says he isn't?

[wantsanswers]

he recently got the messenger app on his phone, and most of the time when he says he is offline facebook says the same thing. but sometimes when he says he is busy doing something, 20 minutes later it will say that he is online, so then i ask him why he is on fb, and he tries to tell me it's his phone. but if that was the case, wouldn't his phone say that he is online all the time, not just sometimes? because when i know he is sleeping, facebook reflects that he is not online, so why would it all of a sudden say he is when he isn't?