I think what they mean is something like this:

I had read the cert terms and had sorted Delta flights based upon the fare class. At the time it returned like 3 different roundtrip itineraries at 3 different prices.

I then went to use the cert, a minute or so later, and it only returned one valid itinerary.

So apparently not all fare classes (ie, T class all the way for example) are eligible.

but this was like 9 months ago so things may have changed. I've not tried something similar since.