How exactly does the paintTexture->stencil->scaleImage(Shift+RMB) workds?

Any time I try to use it, it scales the image differently, one time x and y ( uniform ) the other only x and the other only y.
So sometimes I have to press X or Y when scaling to bring back the uniform scaling.
Isn’t it supposed to always work on uniform when scaling it as default?
Is it a bug?
I’m using blender 2.75a.

When using stencil scaling, X and Y are used to constrain the scaling to one axis. Pressing one of the buttons twice reverts to unconstrained scaling.Because of scaling, it is possible to scale an image to aspect ratios different than the original image and introduce stretching. To restore the aspect ratio of the original image, use the Image Aspect button below the brush mapping.
By default it scales uniformly