Here's the situation....lets say you have a 3pm flight. Your DME p/u time is about noontime. You decide that you want to hang at MK, but you get caught up in a long line at Space Mt...so you don't get back to Pop until closer to 12:45. Obviously, you have missed your assigned bus.
BUT....no one at Pop has been scheduled for a p/u until 2:00...no one had a flight scheduled that necessitated a pickup before 2:00. So, no DME bus coming to Pop. Now, you can call a cab to get you to MCO.
Or....you get back at 12:45, your bus has obviously come and gone. There is a bus arriving at 1:00, but it is a full bus with no room for anyone else...so, you can't get on that bus. Maybe you can get on a bus if one is coming 30 mins later, maybe not.
In all seriousness...you are assigned a particular bus based on your flight departure time. You can not assume that you can just 'hop on' another bus. You can't expect to displace those who were booked on that bus. So, plan on being back at your resort no less than an hour prior to DME pickup time. You will need to be on the platform, ready to go (bags in hand, the entire group there) no less than 15/20 mins prior to the stated p/u time. that time is the time the bus is scheduled to leave the resort, not when you should think about getting to the bus stop.
Get all your bags from Luggage Assistence...assuming you stored the carryon bags before leaving for the morning. Use the restrooms, grab those last minute snacks or gift and head to the bus stop.
It's really important that you use the bus you have been assigned to. Otherwise you can just get a cab...at your expense. Of course, if you do that, and you have any/all carryon bags with you, you can leave directly from the park rather than take the time to go back to your resort...giving you an addlt hour at the park.