Keyword- plural vs. singular

My listing is Amazon’s Choice for my main keyword as “word1 word2 word3s” but doesn’t appear at all if the keyword is singular. What’s up with that? I’ve been #1 in this small niche for over a year, starting entirely organically and briefly having some PPC that wasn’t worth it. I just put on PPC for the singular one as manual-exact to try to fix it, but I’m still baffled. The item is sold as a pair but something that anyone might search or sell as singular or plural pretty equally (like napkin or napkins).

Also, in a big brain move, I’m conveniently #1 for the singular version too, just with a different listing for a different product of mine that I launched more recently as a back up. So I might not be losing too many sales because of it, but still.