I suggest that BHX is a more European business oriented airport, than some - and also has a high concentration of traffic to and from the Indian subcontinent.
European business is at it's slackest in August, and loads to India and Pakistan are also poorer during the summer months as the climate is at it's least pleasant.
Add to those factors the windfall from the BA fiasco at LHR in July, and it goes some way to explaining the comparative small increase over July.