Easter egg: The Office (UK) Series 1 DVD, on disc 2: Go to "Deleted Scenes" and choose "'Slough' by John Betjeman". Before the scene plays, there is a description page with a flickering fluorescent light. The screen will go dim for a few seconds just before the deleted scene begins to play. Press 'enter' or 'play' at this point, and the "Who Cares Wins" video from episode 4 will play.
Easter egg: On the series 1 DVD, allow the main menu to come up on screen. Wait for the menu 'loop' to come to an end. When the menu starts up again, a phone will ring. Press select to get a full length version of 'Free Love Freeway' which was shot for the episode 'Training.'
Christmas Special: Part 1 - S3-E1
Easter egg: Go to the special features and go down to 'If you don't know me by now.' Wait until the photocopier starts copying, then press LEFT.
Answer: It's because he keeps doing it, like clockwork, on exactly the same line and he has such a blank, serious expression on his face that Tim (Martin Freeman) can't help but laugh.