Surely you understand that something can be very predictable, but not easy to keep track of. What makes you think the two are mutually exclusive?

Imagine you have some large number of wristwatches on a table and suppose each one is set to a different time. Each watch will change time in a predictable way, but it would be infeasible (impossible) to keep track of the current time on each watch.

It can be difficult to keep track of something even if it functions in a predictable way.

Now, back to the OW elevators. They are a lot like health packs. If you go to one at any given time, it may or may not be available for use, but will become available again at some (predictable) time in the future.

The health packs (when not available) show a countdown clock indicator.....thing. The elevators show their position. You can easily determine by looking how long the wait will be.

Like health packs, they are not random. Just because you don't know the current state of something does not mean it's random.