If you give some more information about your travel (departure date and time) it will be easier to answer your question.

The actual reservation for a couchette in a 6-person compartment costs €34 (book on tickets.oebb.at) since Interrail charges extra.

The price you found at NS is not for a couchette but for a seat, since you mention that a reservation costs extra. That means you're not comparing the same things. If you only want a seat, that can be booked for 3€ via ÖBB.

Thank you so much for the quick reply!

That makes a lot of sense.

and just note the 39€ is not refundable or exchangable :/

But in general: yes, there certainly are cases, not that few in fact, where a passday+RES-cost is more, or at least equal to the advance cheap price (normally/often only off-season as special promotion). F.e. there is now-here in NL-promotion for RETurn to LON by €* for 79€-whereas the supplmt to pay with a pass for this is 34€. DB often has special Sparpreise for IN DE for/from 19.90-also overnite.

In all fairness it should also be pointed out that a passday (as total cost : nr of days) is not the cost if you compare f.e. a 5 or 7-day pass for those 2 days.

When I try to delete my reservation and see the refund policy it says the following:

CHECK REFUND POLICY

100% refundable up to 15 days before departure; 50% refundable (with a minimum of 15 eur) from 14 days up to 1 day before departure; Not refundable from the day of departure. Partial cancellation of a ticket with several passengers is not possibled

I stubbled upon this cause I searched in the reservation planner tickets from Amsterdam Central to arrival Florence S.M.N. It only showed me one direct option for a night train from Amsterdam to Zurich which had only the 6-person couchette available, followed by other connections ending up in Florence. I purchased these tickets, but then later i searched for tickets just between Amsterdam and Zurich and suddenly it showed me an option for regular seats in another night train, but exactly same time, from Amsterdam to Zurich that wasn't visible earlier when I searched for trains between Amsterdam and Florence. I didn't select the option ‘no reservation needed’ earlier. Cancelling the reservation now doesnt make much sense if i only save a few euro for a chair instead of a bed. I'll leave in a few days.

but i don't undertand why it didn't show me the other train with regular seats in my first searches from Amsterdam to Florence

The problem is the train with the Beds & Couchette and the train with the seats are in reality the same train (just different Trainnumbers). 
On Few Routes like Amsterdam - Zürich the Railcompanies work just partly together means the Sleeper and Couchette Carraiges are operated on behalf of just one or two companies and the seating carriages are operated together by all companies along the Route :) 

eg. The Sleeper and Couchette are operated by Nightjet (Austrian Rail) on this Route which means Tickets of DB are not valid on this service. The seatingcarriages how ever operate as a Normal Intercity with almost the same trainnumber (Nightjet 402 Intercity 60402 ) and on this Intercity you can use then your DB tickets. It´s a confuseing system and they invent every Year new ways how to confuse Customers.