I think I've finally found the reason why it's been doing this. Every time you click the item it generates a preview the very moment you click down on the item. During the period that your mouse is clicked down, it runs its image anti-aliasing algorithms to create the preview.
I've noticed when I click on the item as fast as I can (ensure that I release the click as fast as possible), I get a better probability of the image being aliased rather than smoothed. Being a programmer, I think it's definitely because the computer can't generate the smoothed preview fast enough if a fast click is used. So to always get aliased images, I'd think a macro is probably the only way to click fast enough without having to rely on the speed of your finger every time you click.
(usually you can set the execution time of the macro, this is the only thing I can think of to solve the problem)