It only shows the decimal if the unit isn't full strength. The 'K' (and 'M' if you ever manage to get a million troops) is for visual clarity and to more easily distinguish fleets and armies. Judging from our experience with testers and betas, after a very short period of 'this looks cluttered' people quickly got used to and liked the change, so we'll look into adding options for it if anyone actually still cares a while after release.
The pictures from the diaries shows the decimal with units at full strength??? on picture austrian army is 19.0K... But I´m not the one thinking it´s a totally bad move