I noticed this a while ago but wasn't sure how to reproduce the issue because it's a bit odd.
The problem appears to be a difference between how the Trading Opportunities page handles the first landing there as opposed to subsequent searches.

For instance, if I want to search for people who want Containment Priest...
If I go to the Trading Opportunity page and specify that I'm willing to trade Containment Priest then I get 6 people who match this criteria. Great.

Now, if I go to the Containment Priest card page and click on the link for "Find users who need this", I end up getting back 21 results. Interesting.

Possibly even more interesting is the fact that, if I change nothing, clicking apply on that same page that just showed me 21 results will instead show me the original 6 that I saw during my earlier search.

When you click on "Find users who need it," then it automatically fills in the "edition" tag wit the latest "core" printing (when there is one, or the latest edition period if it's only ever been printed in supplemental products like Containment Priest or Command Tower) but, and this is the important bit, doesn't actually use that as part of the search criteria. So it's finding all 21 users who say they want Containment Priest, regardless of edition. But when you click "apply" then the "edition" tag is already filled in so it gets applied to the search results this time and only shows the six people who specifically want that edition.

This has got to be a bug, and I think it was alluded to in another recent thread but I'm not sure they saw exactly what was going on, either. The solution is, of course, to either leave the "edition" tag blank when the page first loads, or to actually filter by the edition since it was selected. Personally, I'd prefer the first option. If you fill in an edition, you're limiting the search results to people who specify they want that edition, even if it's the only one there is. With Containment Priest, for example, all 21 people who want it want the C14 version, because that's the only one there is, but if you only look for people who say they want the C14 version then you'll only hit six of them. The other fifteen people are left out of the search, even though they should be perfectly valid hits.

This brings up another issue...if you search for people who need a particular card, and you fill in edition for the search, then it should return people who want "unknown edition" (i.e. they don't care what one it is) as well. Most of the time you're not necessarily asking "who's looking for M10-edition Lightning Bolts, specifically?" for example, you're actually saying "I've got this Lightning Bolt for trade, it's from M10, who wants it?" Those might seem like the same question, but if one guy wants "M10 edition Lightning Bolt" and another guy wants "any old Lightning Bolt you have", then they actually both want my Lightning Bolt from M10, even though the other guy didn't necessarily say so. Limiting the search to people who want a specific edition, just because I filled in what edition my card was from (to avoid the third guy, who's only interested in ABU Lightning Bolts) isn't helping anyone find any trade partners.