Interesting graphs - I assume the reason that the number of staff overall has increased is that the WTBTS have focused their resources and expanded the world HQ in New York & largest branches in countries like the UK , Mexico & Germany. The Bethels that closed have usually been smaller branches which sometimes only had a handful of staff.
I know the WTBTS has culled a lot of surplus staff but this will always be a difficult process and those who know how to play the system will manage to stay in Bethel. I'm related to a couple of "old stagers" in London Bethel who basically don't really do anything other than maybe a bit of cleaning or paperwork occasionally but would still be counted in the head count. I remember asking one of these if he was ever worried he would be "reassigned to the field" and he just tapped the side of his nose and said " you need to have contacts , boy! - we'll never have to leave".
The overall doubling of costs is a bit more puzzling but as the WTBTS doesn't produce audited accounts in the USA its always impossible to know for sure. Maybe increased medical costs etc.