Basically, LL controllers can now climb the CPT easterly departures to FL80 (1013hPa or more) or FL90 (less than 1013hPa) without prior co-ordination with London TMA controllers. So now the LL guys and gals have the confidence the departures will be "up and away" sooner, they will 95% of the time turn them 'on the inside' of the downwind inbounds, which involves them being turned on to a heading of 270-290 degrees shortly after making the initial right turn out. When the LL controllers didn't have the option to climb higher than 6000' (without prior co-ordination), a much higher percentage would be taken further south before being turned 'on the outside' of the downwind inbounds, thus a greater spread of the noise.
That's what I think anyway. Apologies if this has already been explained/expressed before.