There are a couple of trackers open on this. See here and follow the web of other issues attached/related to it.
As to an explanation, I remember reading on a Moodle Doc that gave an explanation. At this time I haven't been able to locate it though. The simple answer is that teachers will always see all grades; regardless if they are hidden. This way they have the most up to date in the gradebook. If the tracker is solved in the future, that would give you the behavior you desire though. So at this time, I would vote for the tracker to help push it up the development list.
Happy 
oodling!