Sometimes you may encounter something that looks like a scratch on a tablet. But it is what I call a "false scratch" or a "temporary scratch". I use these terms because, these kinds of "scratches" can be removed by cleaning the tablet or even rubbing the it away with your finger.
These marks are caused by the pen displacing material on the surface. This material can include includes:
Oil from your skin
Dust and other grime
Tiny bits of the nib that have worn away.
Here are some options for cleaning your tablet: Cleaning a drawing tablet