‘They’ - be it the prod team or whoever else that compiles the credits - aren’t even consistent.For Sept 25-30, the cast is listed alphabetically by forename, while for Oct 2-7 it is done by surname, but randomly within the infinite number of Archers, and using a special alphabet where F follows G.
Short version: the names are in billing order first on the left, the photos are most prominent character (which is often but not always the top billed actor) in the middle. Also if some of the names are wrong at that point they usually shuffle the others about so that they’re all wrong.