Complex sentence: Despite these numbers, all of the clergy were not present at the services--absence was an enduring fact of life in all cathedrals in a period when those seeking career advancement would accept prebends from more than one cathedral.
Split into simpler sentences: Despite these numbers, all of the clergy were not present at the services. Indeed absence was an enduring fact of life in all cathedrals in a period when those seeking career advancement would accept prebends from more than one cathedral.