Actually, I've noticed that in recent months in my area, Shuttle Express is building in an extra 20-minute cushion.

They used to say 'Driver will be there at 4:30'. Driver was always exactly on time.

Now they say 'Driver will be there between 4:10 and 4:30'. Pickup time on the reservation is definitely shown at 4:10. If you get nervous (based on past punctuality) and call at 4:25, just to make sure there's no problem, dispatch will say "it's not 4:30 yet", and sure enough, the driver gets there by 4:30.

I think the extra 20 minutes is to be certain you've checked all the doors, bags zipped and outside, hat and coat on...so you're actually ready to go, not ready to start getting things together to go.