You might see a message on your TimeToSignUp sheet that an item is "temporarily unavailable." This is due to someone else currently being in the process of signing up for the item.
When a participant checks the box for an item, that item is "temporarily unavailable" while the participant enters his/her personal information to complete the "sign up" process. If the participant completes the "sign up" process, then the item is claimed by the participant. If the participant does not complete the "sign up" process, the item will return to being completely available, usually in 8-10 minutes.
You can think of this process as a "shopping cart" where someone has put the item in their shopping cart and is in the process of "checking out" but hasn't yet completed the process.