Why the rotation of two equal surfaces (with different shape) do not give the same volume I will explain my question with an example:
Let's take two surfaces that have the same area:
SURFACE A
$$ f(x) = x, x \in [0,6] $$

SURFACE B
$$ f(x) = 3, x \in [0,6] $$

Both surfaces are equal:
Surface A, Sa = 18
Surface B, Sb = 18

But now if we rotate those two surfaces around the (for example) x axis.

We obtain two different volumes:
Volume A, Va = 72*pi
Volume B, Vb = 54*pi

I've no problem to apply the formula, but I found the result a bit counter-intuitive. We apply the "same" area (with different shape) around an axis an we obtain a different volume. 
Why is that (intuitively) ? 

There is a result that says that equal areas rotated about axes an equal distance away from the center-of-mass (or centroid) of the areas, produce equal volumes. If the area is not distributed equally with respect to the distance from the axis of rotation, you will get different volumes.
