I think that developers should better explain the 20 minutes rule.
The only thing I can think of in your case is that if you have more than two stops on the line (as I imagine from the screenshot), then the frequency number you get is referred to the total trip (back and forth). However, customers are not forced to use the whole line, but may hop in a stop and hop out the next. In that case, the travel time between two stops (in one direction) can be under 20 minutes and satisfy the 20 minutes rule.
By the way: why in your calculations you get a different waiting time for the different stops ?