Because the cogs exert friction on each other which is proportional to load as they transfer the power.

Place your hand on the desk, press down gently and then slide it along the desk.
Now do the same thing but press down harder, still the same hand and the same desk, but now with a greater loss.

Essentially that mimics what happens with the cogs they are pushing into each other harder with more load.

If you have access to the graph for one of the cars that made around 350bhp, I would wager that it will be showing losses of around 90bhp on the figures you were given on the day.