Why do some people get waitlisted when there is inventory in stock?

When you are doing a live sale with multiple shoppers commenting on an item that's nearly out of stock, some of your commenters will get this waitlist message after their comment:

And if you look at your inventory level in Shopify, it might be positive. The reason this happens is due to the checkout hold time.

The first person who commented has it held for them at checkout. The inventory is thus positive since it's being held for her. But others who comment after her can't add to their carts until the first commenter's checkout hold time expires.