Think radius, not diameter. (Since that what is being simulated by trapezoidal scatter values.)
The value of 2 is increased by 25% twice, separately. Once for each direction on the axis. 25% of its base value is added to each end, which adds .5 to each side of 2. making 3.
(Now I don't know if this is how the engine works, but it's how the math he's doing works.)
Let's do concrete values. Let's say that horizontal scatter is 1 on each direction, and vertical scatter is 2 on each direction.
That gives us a square which has size (|-1| + 1) * (|-2| + 2) = 2 * 4 = 8
Now, we increase all scatter values by, say, 25%. That means that 1 gets displaced to 1.25 and 2 gets displaced to 2.5. The area of this would be:
(|-1.25| + 1.25) * (|-2.5| + 2.5) = 2.5 * 5 = 12.5
Thus, the net increase is 56.25%.
Alternatively, you could have calculated the net increase directly, by simply raising the 1-D difference to the square (1.25 = 1.25 = 1.5625
In the case of your trapezoid example, the 2 is the diameter. Thus, 1 is the radius. Thus, a 25% increase will still increase both the radius and the diameter by 25%.
Regardless of whether you multiply the whole value by 1.25, or multiply both halfs by the same amount and them add them together, the result will still be the same!