Anecdotally, I’ve run into this “issue” when trying to book 2 seats on the same flight under two separate PNRs (usually paying cash for my ticket and using points for my husband). If the site says “1 ticket left at this price”, I can get the first ticket at $X, and the second ticket is $X+Y (or the points equivalent). If I try to buy two tickets on the same PNR when the site says 1 ticket left at a specific price, it’ll push me to the higher fare bucket.

That being said, revenue management is a complex and proprietary beast and fares are adjusted as needed and not necessarily in a linear fashion.