It can certainly be confusing when you immediately respond to a signing offer as available, but find that it's already been given to another signing agent. How did that happen?
The answer is that there are many things happening simultaneously and very quickly. Notifications are being sent to signing agents, contacted agents are putting in their response, and the hiring company is evaluating these signing agents and their responses and making a hiring decision. These events don't necessarily happen one after another.
When you see that the signing's already been given to someone else, what most likely happened is that during the time a notification was being sent to you and you were responding, the company made a hiring decision. There were likely signing agents who were contacted before you were notified. Contrary to popular belief, signing agents in an area are not notified all at once.
For a more in-depth and visual explanation of how this situation can arise, please take a look at the video below: