The difference between a basic wheel and a good 60 mm aero wheel is about 200 g of drag at 30 mph and yaw angle 10% (I'm using 30 mph because that's the standard testing speed for aero components).
50 grams of drag reduction = 6.5 watts of power savings at 30 mph, scaling as speed cubed. So, Two aero wheels are equal to extra 15 watt at 20 mph.
The chart definitely does some fudging, 15 watt do not translate into 1.1 mph at 20 mph in the real world. For starters, they seem to be using very low numbers for overall aero drag - Cd*A of 3.4 ft^2 is in the ballpark for a time trialist on aero bars, but would be hard to achieve on a road bike. And there's at least one other source of discrepancy. But it's not unreasonable to expect an increase of 0.5 mph.