B777 fuel jettison
 

FCOM says for a -200 5200kg will be left in each main tank. For a -300 it says 3800kg will be left in each main tank.
Anyone know why there is the difference?

Which engines do you have on the - 200? Could it be that?

It's a difference that was introduced in production several years ago- on our fleet newer-built aircraft are 3800kg and older ones 5200, even within the same variant.