For each flight, an airline decides how much inventory to make available in various “fare buckets” based on how full the flight is, how close to departure it is, and other factors (like whether they feel they need to reserve seats for an over sale or IROPS situation).

”one seat left at this price” means “one seat is left in this fare bucket right now”, at the price displayed. It means it is only currently possible to book one seat at the price shown, but it doesn’t mean that more seats aren’t available at a higher price. Also, it doesn’t mean things can’t change - seats are often held temporarily during the booking process to avoid selling out while someone is checking out, for example; this can cause prices to temporarily spike until the availability is released (explaining some of the behavior people often wrongly attribute to “clearing cookies” or “using private mode”).

Availability changes minute by minute, as do fares, so if you don’t like the current situation, you can always hope more seats are released in the future for a low fare bucket...