As wiggy says. It's happened to me quite a few times (not RYR btw) and only once did we have to actually await our original delayed slot time. When the crew report "ready" to ATC, a "ready" message is sent to the flow regulation unit. There's always a chance that another aircraft (not necessarily at the same airport) has had to relinquish their slot for some reason or other. That slot will then be re-allocated to an aircraft with a later slot but which is ready to go at short notice. And so, you slowly move up the line.