The listed unrestricted slots don't directly reflect how large an event will be.
Those are just what remains open for any team, anywhere in the world to register for without restriction.
After these open slots get taken, teams are added to a waitlist and many if not most will get added to the event later.
If you end up on a waitlist it's important to communicate with the responsible Regional Director or District Committee to follow the progress of your teams' request.
These are why it's important to register for your favorite event as soon as possible.
Each event typically reserves ~10 slots for late registering new rookie teams.
Unclaimed rookie reserved lots will eventually get assigned to waitlisted teams.
Rookie teams typically finalize their organization during the Fall school semester and about two thirds are not ready in time for a late September registration.
In addition, especially when an event is very popular, the Regional Director may ask FIRST to reserve additional slots, over and above the rookie reservations, for late registering local veteran teams. Some veteran teams my be changing Main Contacts, reorganizing, or those who need school board approval that doesn't come until later in the semester.
Events with zero open slots include World and District Championship events where teams must qualify by points earned during regular qualifying events.
Also events where teams go immediately to the waitlist where a vetting process is required before being admitted to the event, e.g., the Seven Rivers event is restricted to a particular part of the FRC world for Championship logistical reasons, because the event is the last regular season week.
By the way, here is an easier way to see all the remaining open slots at all events:
http://173.255.246.196/2017/