On the 777 it seems to be a mixture of two things: occasional inaccuracy in the FoB measurement mid-flight when partially loaded, and rather optimistic fuel predictions from the FMC.
The good news is that it seems to get more accurate when you’re down in single figures. I’m not sure what level of detail the FMC goes into on the approach, but you often see 30-40 track miles for 100kg burn which obviously isn’t right, especially as you dirty up on finals.