Based on my limited experience ordering from Walmart's website (have bought only a few things there over the years) I think that maybe for an item that's selling quickly, instead of keeping the listing up but marking it "out of stock" when it's out of stock they temporarily remove the listing.
I remember years ago buying an electronics item that was hard to find and frequently sold out - it would come and go on the Walmart site, you had to grab it while it was there if you wanted it.
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