When I make textures, depending on the texture, I may import it in non-square aspect ratios that are 32, 64, 128, 256, or 512 on one direction or the other, doing the rescaling in Photoshop before importing it.
Once in a while, that still results in the aspect ratio being changed on import! Even when the pixel counts are exactly on one of the above values before saving! I imported some textures a few days ago where the 512x256 textures came in rectangular, while 256x512 textures got streached and became square!
In fact, come to think of it, I think that ANY time the width is less than the height, SL is importing it with the height dimension as the final form in both directions. Can anyone verify that? See if importing a precisely 512x256 texture comes in rectangular, while 256x512 textures gets streached to became square. I think it will... But they should both import as rectangular.
