I had a discussion with a sofer the other night and he wanted to say (after I kept using the following phrase in describing certain changes in a tzuros haois) that it's not always right to use the words "kosher b'dieved" in terms of describing an ois that wasn't written *exactly* like it should be. He said rather say "kosher" because that's what it is. Maybe this way it's not "mehudar" but v'dai kosher and not just kosher b'dieved. I've seen lashonos before in MS that sometimes says kosher and sometimes say kosher b'dieved...Perhaps that's exactly what the chiluk is. One is "Kosher" (changed somewhat from what it should be, just not "mehudar") and sometimes could be "Kosher B'dieved" which is certainly not "mehudar" and a darga less than kosher -- but still kosher enough to use. Could be there is even a dargo of kosher and mehudar just not the "lichatchila sh'b'lichatchila"...I suppose this is more negia to a socher but certainly when a shoel comes with shailo and will ask afterwards of it's still "mehudar" the rov should be able to say.
Perhaps the olam can share their thoughts or knowledge in this. A good example of the chiluk would perhaps come to clarify more.