The wording on that is a little odd - but not unexpected given that it's likely a German -> English translation of the fare rules.
As you're probably already aware, airfares - even on the same route - can vary dramatically depending on a number of factors, including when the ticket was bought.
Let's say you bought a ticket for a flight that cost 1000 EUR, and now you want to change it to a different day.
If the flight on that new day now costs 1200 EUR, then you will need to pay the "fare difference" between what you've paid and the new fare, being 200 euros. Based on the rules you've quoted for "Economy Flex", that 200 Euro is all you'll need to pay. With "Economy Classic", you'll need to pay 70 Euro PLUS the fare difference, for a total charge for 270 Euro.
Now, what if it happened that the fare for the new flight you wanted was only 800 EUR? The fare difference in this case is 200 EUR, but it's 200 euro cheaper, which means that (if the fare rules allow it) the airline owes you 200 euro.
From the rules you've quoted for Economy Flex, you ARE able to get a refund of that amount - minus a 70 euro fee. So you'll get a 130 Euro refund.
The rules for Economy Classic say no refund, so whilst you would still be able to make this change, you will not get any money back from doing it, and in fact would most likely still need to pay the 70 Euro fee to make the change!
If you wanted to cancel your ticket it's likely the same conditions would apply (Economy Flex you could get a refund minus 70 euro, Economy Classic no refund), but that would be covered in other entries in the fare rules that you haven't quoted so I can't say for sure.