The 1614 X20 from Durham to Sunderland is formed by the 1502 20A from Sunderland (and hence arrives at 1557). That's 17 minutes layover between arriving in on a 20A and going back out on an X20.
This bus then has 10 minutes layover in Sunderland prior to working the 1710 X20, and another 13 minutes layover once it arrives back into Durham again to form the 1809 20A...
Difficult to justify much more layover than that!

This must be a recent thing too, as I used to get that X20 regularly at the start of the year and it was always on-time.