A room becomes available to guests who are checking when housekeeping finishes cleaning that room after the prior guests depart.
Let's say you've been assigned to room 1621 (an arbitrary number that I just made up). The prior guests check out at 7:00 a.m. to catch a flight. Housekeeping is able to get to it soon after that. You get to the front desk at 10:00 a.m., and you can go directly to your room.
Now let's say you've been assigned to room 1621, but the prior guests didn't leave till 11:00 a.m. Housekeeping is swamped with rooms from which guests checked out between 10:00 and 11:00 a.m. They don't get to room 1621 until after 3:00 p.m. By 4:00 p.m., it's marked as cleaned, and you can settle in.
When you check in, you can always ask for a different room. If the resort is at less than 100% occupancy, there may be some clean, unassigned rooms -- unless other guests asked for them before you did. However, those rooms probably won't reflect whatever preferences you requested when you made your reservation.