This is indeed surprising! For those like me who need a bit more detail before this clicks:
If you are going X miles, the difference in gas usage between A mpg and B mpg is (X/A)-(X/B) = X(B-A)/(AB). So in all these pairs, the constant is (B-A)/(AB), in this case 1/60. So each adjustment means your fuel usage goes down by 1/60 of a gallon per mile.
One faces a similar problem with thinking about how much time is saved to go a certain distance: velocity is measured in meters per second, not "slowness" in seconds per meter.
Thus speeding by 10 miles per hour makes much more difference in your time if it happens at 20mph (3 minutes per mile -> 2 minutes per mile) than if it happens at 60mph (1 minute per mile -> 0.86 minutes per mile).
This can sometimes be erased because one typically (at least in the US) spends more time at the highway speed, but if we're talking about you need to spend 10 minutes getting on the highway, 30 minutes driving on it, and 10 minutes getting off it, then even allowing for 5 minutes of the city driving to be non-speedable (it's spent stuck at three traffic lights, say), you can save 5 minutes of time by speeding 10mph on the only 5 miles of city streets, but only 4 minutes of time by speeding 10 mph on the 30 miles of highway. You're speeding the same amount for 6 times the distance and something over twice the time, but because your average speed was higher it simply doesn't buy you as much.
Working out the numbers also helps you realize that speeding on the highway to get a substantial amount of time is, while not pointless, more unsafe than you think. Even under great circumstances, like if you are facing 40 minutes of driving -- if we're talking about a 70mph highway and you are 15 minutes late while you think 5 minutes late is still socially acceptable, you need to cut 10 minutes and thus average 4/3 * 70mph = 93.3 mph to make that happen. That means that to handle the moments where you are stuck behind two cars both going 10 over the limit at 80mph, you will need to at times be going 30 over the limit. And that's with a relatively long commute! If it's a 20 minute commute you have to drive this recklessly just to shave 5 minutes.
If you are going X miles, the difference in gas usage between A mpg and B mpg is (X/A)-(X/B) = X(B-A)/(AB). So in all these pairs, the constant is (B-A)/(AB), in this case 1/60. So each adjustment means your fuel usage goes down by 1/60 of a gallon per mile.