Keeping to time all the time and having no bunching of frequent services like the 63 is pretty impossible, to be fair to Arriva.
There's the level crossing in Redcar that can hold them up both way, as well as all the traffic lights and usual congestion on the roads. And we've all seen buses running late only to get to a stop only then for the boarding passengers to ask questions, look all over for their money or pass, and basically delay things even more.
So the first bus gets delayed and end up picking up passengers who intended boarding the next bus - which in turn is quieter and can keep to its time easier, ending up behind the earlier one.