I'm not sure. I'm guessing the engine handles scaling differently. I believe there are loads of different filters & so on that apply to scaling? In photoshop you have all those bicubic scaling this & that options available when you use the image size tool. VS uses interpolated or nearest neighbor scaling, if I remember correctly.
I guess I'm not really the most qualified person to be answering this as I'm no artist & I've not really got much insight into the inner workings of the engine. Simon?