For many studies, we invite more candidates than there are spaces to account for the likelihood that some candidates will be unavailable. In order to ensure fairness to every participant, we send out sign up emails to every approved candidate. It only takes a few people acting quicker than the others to fill those slots.
We know this can be frustrating. In order to ensure that you have the best chance to sign up, keep a close eye on your email inbox for invites to sign up and act on it as soon as you get them!