Children/teens not staying at their desk to do their homework

Hey,

Very stupid "issue", it's really not a big deal but I'm really curious to know what's happening in these sims' head.

I like to have children and teens do their homework at their personal desk, but since they don't go there naturally, what I do is that I ask them to sit down and put their homework in front of them, THEN I ask them to do their homework. Sometimes they just open their book and start working, but sometimes they take it and go somewhere else. Is there a reason for that ? 

Sorry for the useless question x3