Okay, so the first time I posted this I was definitely not thinking straight at all. Here's a correction.
So let's take a look at the math involved:
In a scale, pixels (x',y') are given by the following:
x' = x * x_scale_factor
y' = y * y_scale_factor
In a skew, pixels (x",y") are given by the following:
x" = y * x_skew_factor
y" = x * y_skew_factor
If you remember our image is partly scaled and partly skewed. Remember that the images are divided along the line f(x)=x meaning along this line pixels of the skewed picture approach the scaled picture.
So let's pull out that high school math and we'll find that means that:
x * x_scale_factor = y * x_skew_factor
y * y_scale_factor = x * y_skew_factor
By substituting in the line y=x this tells us along the line y=x pixels converge when x_scale_factor = x_skew_factor and y_scale_factor = y_skew_factor.