Unity is rescaling my texture when I try and import it?

Hey everybody, I'm working on a Unity project for class, and I'm attempting to import some of our spritesheets right now. The sheet is 512x32 (note POT), and when I drag it from my windows folder into our project it shows up as that correct size in the preview window. However, when I change the Texture Type from "Default" to "Sprite (2D and UI)" so that I can chop up the spritesheet into its constituent sprites, Unity changes the size to 600x32 (note NPOT).

The ONLY import setting I am changing is Texture type, from "Default" to "Sprite (2D and UI)", and it still resizes the image.

This seemingly unjustified rescaling then ruins some of the sprites, so I'm not sure what to do or how to fix it. I've tried fiddling around with many of the import settings, so far with no success. I haven't found anything else about this online, so any help or advice would be greatly appreciated.