the difference is, it doesn't take the same energy. In the kinetic energy equation, look at the V^2 term. If you start at velocity X, and increase your speed A, your resultant velocity component will be (X + A)^2. Multiplying that out, you get X^2 +2AX + A^2. To go from 0 to 10 mph, you have to add 2 * 10 * 0 + 10 ^ 2 = 100 units (I don't remember the American unit applicable here). To go from 90 to 100, you have to add 2 * 10 * 90 + 10 ^ 2 = 1900 energy units. It takes 19x as much energy to increase speed 10 mph at 90mph IGNORING DRAG.
The X^2 gets ignored because we're only considering the amount of energy we have to ADD to go from X to X + A, and the KE at Vx will end up canceling out the X^2 in the resulting equation subtractions.