For single levels the game shows how the stats work and it updates whenever beaten, but the overall score doesn't change and is never the same points as the sum of the missions, nor is it congruent across different people (even if it adds points based on the difficulty it doesn't make sense as for people that have both done it on normal it adds different number of points on top of their sum of the missions).
Probably counts many things together which might not be listed seperate.
It's like the general XP you get, the difficulty, the completed assignments, maybe even dogtags and weapons you collect during the Campaign give you a little extra and maybe even if you reload at a checkpoint or you die it could still count the XP you got in the section where you actually restarted then before again, for the overall XP.
Maybe even if you replayed several missions or just parts of it, to do something might take it to the overall XP.
It's hard to say as it can't be really followed afterwards.
Could also be kinda bugged in Battlelog too.
Greets,
Travis
