First the the cycling of the mold from accepting the molten plastic to cooling enough to remove the sprue from the maching. Even with the best steels, that constant delta will cause some level of distortion over time, which produces flashing and cause a certain waviness in the flat sections, and can exaggerate sink marks depending on how fast the tool is cycled and cooled.

Surfacing the metal facings of tool can stop the flashing, but it's not long before that will affect critical dimensions.

Also, it is customary before running a tool that has been in storage to make a pass at the tool shop with a polisher and diamond abrasive paste in the cavities. That tends to remove the sharpness of the engraving.

Compare a '67 GTO annual to what is in the box today... you can see the difference. While you can knock one together with a tube of Testors, it will be no end of frustration to anyone who has built a really great kit.