I visited the station a few years ago and it is the reason for my original comment, at the time this was explained in some detail by one of the people on the tour. Very impressive by the way and highly recommended if you're in the region.

As for the bookkeeping:

A certain amount of electricity is produced using renewables which is always < the current baseload. It is then (much) more profitable to sell this energy to consumers directly and then to use the reduced base load requirements to top off the lakes before reducing base load generation capacity. At night the process reverses and now the 'newly minted green electrons' get added to the green energy already being sold. This makes more money than selling the nuclear/coal/NG generated electricity directly even though there is some loss from the whole pumping operation.

Note that the scheme already made money based on the demand pricing of electricity, the 'green' aspect simply made it more profitable.