Availability isn't linear; availability displays don't represent the equivalent of how many tins of baked beans of various flavours are sitting on the supermarket shelf.

You could in theory book one seat in the lowest available class and immediately see availability numbers plummet across many different booking classes, depending on the airline's algorithms and revenue management strategies, and how your one single booking has changed the actual commercial performance of that flight to date when compared to the predicted / theoretical commercial performance.