How do type classes make ad-hoc polymorphism less ad-hoc?

The title of the paper that introduced type classes is "How to make ad-hoc polymorphism less ad-hoc".

It seems the type classes approach is being compared to how OOP does ad-hoc polymorphism.

As far as I can tell, the paper never explains how type classes are less ad-hoc than OOP techniques like V-tables or prototype chains, or even has any comparison at all of the trade-offs between the two approaches.

What is non-ad-hoc about ad-hoc polymorphism via type classes?

Note: this is a somewhat objective question, as "ad-hoc" is here a technical term. From the paper, "Ad-hoc polymorphism occurs when a function is defined over several different types, acting in a different way for each type."