Blurred sprite borders - Quick question

This is more of a curiosity thing than a "i really need an answer" thing (moreso because I found the workaround). Anyways... Why is it that when I use a sprite atlas or a sprite with empy borders like this:

It will be rendered fine, but when there's no empty border like this:

then it's border pixels will be blurry? What kind of computation is responsible for this? Does it have got anything to do with them being low resolution sprites? (Being more specific, think of these two images as the ACTUAL .png files)