As /u/lukevdp said the article is correct. Part of my job involves pricing our products and working out rebates when we use 3rd party agents for example. It's surprising how many people struggle with understanding this isn't as simple as they think.
For example if an agent wants a 10% rebate we need to calculate the markup from our standard selling price to accommodate this. If we sell a product for £100 then we'd need to invoice it for £111.11 because 10% of that brings our real selling price back to £100.
If we mark it up by 10% we'd invoice out at £110, the 10% rebate to the agent would be £11 and we'd have sold for only £99.
Comprehending that you don't use the same percentage up and back down is too much for most people here. They eventually get to understand that they can't do it that way, but actually doing it correctly is a dark art.
I had a colleague once who bought a pack of cookies that said '10% free' or something like that. I turned out though that only 10% of the initial volume had been added (hope that explanation makes sense - it's a variation on the issue Steve44 is describing). Anyway so he wrote a letter to the company pointing that out, they send him back an apology letter and a box of free biscuits.
Moral of the story: pay attention in math class kids. If you don't, one day you might have to hand out free cookies.
