Why are sales commissions a variable cost?

Sales commission is a variable cost because the amount of the account is subject to variation.

Think about it:

A used car salesman is paid a commission say of $500 for every car he sells for the month of October.

If he sells only 2 cars, then the sales commission is $1000,

If he sells a whopping 12 cars, then the sales commission is $6000!!

Notice the variation in commission??

This is why it is a variable cost - because it is not a fixed cost, which you know regardless of what happens during the period.!