It only ever happens at the first stops, eg. Camberwell Green & Kings. I've always got the impression that the buses are due to leave the bus station at Camberwell at a certain time, which gets posted on the countdown board as say 2mins at the first stop, 4 mins at Kings. Then for some reason, the bus doesn't actually leave the depot when it's supposed to. Hence, 2 mins becomes 5 etc. It doesn't usually happen further along the route, once it is known what time the bus has actually started it's run. This has been going on for ages, so the problem obviously lies with either Camberwell bus depot and/or Abellio & the info they are sending out.