I'm quoting here from another forum :-
13A double socket rating to BS1363
BS1363-2:1995 requires for double socket outlets that both socket outlets have loads applied via test plugs, 1 test plug having a load of 14 amps whilst the other has a load of 6 amps, making a total load of 20 amps on the cable supplying the double socket outlet. The double socket outlet is then subjected to this loading for a minimum continuous period of 4 hours or longer until stability is reached with a maximum duration of 8 hours (stability being taken as less than 1 degC rise within 1 h). The test is passed if neither the terminals / terminations, nor the accessible external surface, increase in temperature by more than 52 degC.
This still makes more sense to me since each 13amp outlet (or gang) should surely be capable of a 13amp draw. Diversity would probably be allowed for as we know electrical regulations do take this into account.
Not trying to be argumentative, now I need to know for sure :)