Whatever you advise as a 'vacated level' we can assign 1000FT from it. The mode C we are looking at may be up to 6 seconds old, so it lags behind your actual. Unless you are maintaining we need to knock off another 200FT (for each aircraft) which is why will ask for levels when running close verts.
If everybody did it the same it would be a wonderful world, but they don't, so it probably doesn't matter if it sticks in Kap's (and friends) craw and he has to say it backwards. We'll cope, and exactly half of us will grumble when he says it, and the other half when it's done properly.
If you tell us FL216, that's fine. If we need the hundreds we'll use them, if not we'll record the info rounded-off.
As to why? There is no why, just more hos****e imported from somewhere else apparantly more important.
Another grey area is what you say between 10,000FT AMSL & the lowest useable flight level. I had 3 in a row call me on climb at a mode C of around 105 on Christmas day. One said "flight level 105", one said "10,500FT" the other said "...erm... 10,000FT" AIP tries to pretend such levels don't exist, so I think all three were probably correct.
