I guess your question is about I fares. But, it's really true for any fare bucket. Just because a fare bucket has seats in it doesn't mean the particular market that you are looking offers that particular fare bucket, or the requirements for that fare bucket are not being satisfied (Advance purchase (notably 50 days on a lot of I fares), Saturday stays, etc).

However, if the reverse is true, the market has the fare bucket available but the individual flight in question does not, then that fare will not be offered.

Let's take it to the extreme. CVG-CDG could have I9 but you might have to start in SDF and only SDF (just some arbitrary city) to ever see the I fare. On the other hand, there could be hundreds of cities that offer I fares to CDG and if the bucket is I0 then none of them will get it on that flight.