Seems to me the agents calculate it that way because it works better for them.
10% of product sold is 0.10 x (sales total). End of discussion.
It's a standard problem of "direction" in reporting statistics -- going from a product failure rate of 1% to a product failure rate of 2% is a 1% increase -- it is also a 100% increase (doubling of the rate). The result depends on where you start, what direction you go, and at what you are looking. The careless, the unscrupulous, and the biased, can twist these to make answers that favor them (or their point). Which is why whenever someone presents you with statistics, you need to know where they started from and why they're using their chosen methodology. See a lovely little book entitled, "How to Lie With Statistics" Book link Should be required reading. When I was on my town's Finance Committee, we all read it; it was VERY useful.