Generally textures are scaled to a power of 2 on both sides for performance reasons.
For example multiplication and division are faster if the GPU can assume that you are working with POT textures. Even mipmaps generation process relies on that because division by 2 never produces a remainder.
Old GPU (I don't know exactly but probably even several mobile GPU these days) requires strictly POT textures.
Unity by default will try to scale the texture to a power of 2. You can disable or tweak that setting switching texture import settings
to "Advanced"
.
For more details checkout the doc.
Note that generally you want NPOT textures only for GUI, where you need to control exactly the resolution on screen and mipmap aren't used. For everything in the 3D scene power of 2 performs better.