Truth is that is how most hotels/lodging facilities work. They usually block rooms on check in(at property). First to check in is first in line. Front desk agents have pools of rooms by type, as you check in the computer assigns your room based on room rotations. The front desk agent finalizes your room assignment, and if a request is easily accommodated then they do it.
Some lodging places where the specific units may be owned by the users like a timeshare type of place may book rooms at time of reservation. Blocking rooms at reservation time can be a real pain for the housekeeping staff who have to flip the rooms, since you don't know when folks are checking in. However the value in blocking at reservation time is there for facilities where each unit really is different.
Most lodging facilities do what is called room rotations. When the computer randomly selects a room based on type, it isn't really random. They are selecting the room that is firstly vacant and clean, then the room that has been used the least. This ensures the rooms get an even amount of wear and tear. In time shares or condos where they are owned by an individual, the rotation ensures that rentals are evenly distributed so commissions are fairly distribute amongst their owners.
In general, if you request a specific room, you want to be able to check in early, but not have to occupy immediately. The reason for this is if you arrive before the days check out time, your requested room could still be occupied and most definitely dirty. As folks check out, housekeeping cleans the rooms, getting updates as rooms become vacant at checkout. Housekeepers are assigned certain rooms to clean and can only get certain amount done in X amount of time.
Noon-6pm, the room you want may be vacant, but not clean. By 3pm, a majority of what is needed should be clean and ready for occupancy.
3pm-6pm the troublesome rooms that take a little extra time are being dealt with. If there is a particularly large room flip over, they could still be catching up.
By the time 6pm comes around, it's likely all the rooms that were planned to be clean for the day are clean. A vacant and dirty room likely won't get cleaned at that point even if it matches someones request.
So whatever rooms are being offered after 6 pm are not likely to change.
My suggestion is to arrive around 1-3pm to check in. State that if possible you are happy to wait for a room request to be honored, if it isn't currently available, and that you'll be happy to leave your bags with the bell staff while you go do something else.
Don't expect early arrival as well as a special room assignment. Don't expect a special room assignment after 6pm, and know that room choice will dwindle as the hours in the day pass. It's not to say you can't get your room request if you show up at midnight, but the more full the facility is, the less likely it becomes.
This is how many Lodging operations work. However ever place has their own peculiarities, so your mileage may vary.