why is the 'piece' here different than the 'piece' in piece of paper
Do you mean "kartka papieru"? It can be translated as "piece of paper" or "sheet of paper". "Kawałek" means "piece".
Or even a slice of fish
take a look at this example
why is : THE boy is eating a piece of fish wrong. i believe 'the' and 'a' are articles . i used 'the boy' instead of 'a boy' why is this wrong
It is accepted, it should have worked.
Is ryby genetive?
Yes, as it is "a piece OF fish". I don't recall any situation which would be an exception from the rule that English 'of' needs Genitive in Polish.
Why not "a bit of fish"?
It's an accepted answer, it should have worked.