Sometimes selling certain items + other items had a rule leading to a higher percentage. For example, an external HDD could be included with a PC at a discount, but the salesman gets a commission based on the ordinary RRP. Because of the arcane ways some commissions are calculated, this would raise his margin on the deal overall, leading to a higher commission on the PC as well. Not the intention of the retail company he worked for -- but that wasn't his problem.

Lots of little rules and promotions are constantly coming and going in retail, and my friend was quite single-minded about optimising for them.