Frozo - good question, I'm not sure!
I know that Captains are considered level D for the LEPs housing - think that's in the housing policy somwehere. I'm sure I've seen the others in some policy or other but not sure where I got this info originally.
Fac6 - from what I've seen/heard, it's more about the lower levels of staff in the whole company being in theory less able to give up as much money as the upper levels, particularly, I suppose, when it comes to ISD and HKIA staff. I also presume they didn't want to have a different scheme for us as opposed to the rest of the company, so had to roll out the same process for everyone.