online and last seen?? help!


I would like to know how is it possible that I can see a person's "last seen at…." but never see that he is online.
The other weird thing is that If I see "last seen at 19:07" until 20:00pm…but close the application and open again, I'll see that his "last seen.." status has changed to "last seen at 19:33". How is that possible??

Please enlighten me, this is quite weird and interesting at the same time! lol but I am very curious about this! Thanks