Its just how ebay works with limits, you see this with pretty much every large seller.Looks like the seller is trying to create an artificial impression of scarcity. Yesterday the description said they had 5 available with 6 sold. Today it says there are 4 available with 28 sold. They have 70+k feedback score, so there is a chance I'll get what's advertised.
Unless you stop expanding the business or have very few items you will keep having this issue into eternity.
Listing qty 1000 uses 1000 of your monthly sales limit at listing not at selling.
So instead you put 10-20-30-50 depending on how capped you are and your listing tool shift it around lowering stuff not selling to allocate more to those selling.