Draft didn't work or fluke?

[KU40]

On a recent trip from Kansas to Minnesota I stopped in Des Moines for fuel. Des Moines is exactly halfway between my start and end. For the first half I tried to draft whenever possible, which was about 2/3 of the time. I was behind a pickup, a semi, and a minivan, and tried to stay about 3 stripes behind them. Each of these were generally driving between 68 and 70 mph and I was able to use the cruise for all of them except the semi. When I got fuel in Des Moines, I only got 17.5 mpg. For the second half, I just set the cruise on 70 without trying to draft at all, and achieved 20.4 mpg. I didn't make any more or less stops in either half, and the land is fairly stable over the entire trip (just small hills).

Why did I get such poor mileage drafting? Although perhaps I should say I got good mileage on the second half, as I normally average between 17 and 19.5 mpg when I have driven 70 historically. This is the first time I tried drafting. So the drafting was definitely the lowest number, which one would seemingly never expect.

BTW- on the return trip I filled at the same station in Des Moines, and got 17.6 mpg from Minneapolis to Des Moines and 18.6 mpg from Des Moines to Kansas. For the return trip I also just set the cruise at 70 without trying to draft at all. So the drafting was definitely the lowest number, which one would seemingly never expect.