So the answer to the question asked is: no, buses do NOT leave the airport at specific times. They leave as needed. If a bus fills up, or reasonably fills up, in five minutes, it's not going to wait (for example ONLY) another twenty-five minutes just to keep on some non-existent schedule.
Now, later in the evening, it's true if there are only a few Guests are on the bus and more are expected shortly, the bus may wait longer than the Guests aboard think necessary - but if you're the one just getting in, you'd appreciate it