SV you make a very valid point! FODCOM 27/2005 is the source of the change to standard weights. It says:
For adult males this will be 98 kg and for females it will be 77 kg. These figures include a 3 kg allowance for an immersion suit and associated safety equipment.
I take "associated safety equipment" to mean the lifejackets/airpockets but as you rightly say, these are also included in the DOM/ZFW (and have to be since 19 or 20 are carried regardless of the number of passengers carried). So it looks as though we count the lifejacket/airpocket twice. In practice however, they don't weigh very much so perhaps overall its not a big deal?
HC