How come my friends list seems to lack some of my friend guild mates occasionally?

I'm not talking about a sudded disappearance due to him remaking or joining a game. I'm talking about minutes and all the while I was chatting with him and wondering about the issue . In the guild list I can, however, see him being online.

I haven't paid attention to this before but has certainly happened during the past few days. Perhaps this has already been reported and I missed it in my cursory search. If so, sorry for the dupe.