Not much explanation is needed for this - "assuming something I shouldn't" and "copy and paste, forgot to change it" certainly seem to be the root cause of most of my 'bugs' (after blaming third party libraries, sunspots, etc). Blaming it on "the last dev" is a bit harder when you work solo on projects...
Ultimately the Five Whys doesn't yield much, as the fact is relatively simple human error to blame for most of these (of course in a more fundamental way, how/why human error was able to accumulate along with other issues is key to the Swiss cheese and human factors approaches).
So while on some level it isn't complex nor complicated, any attempt to quantify that on a superficial level for reporting (such as "32% of severity 2 bugs were due to configuration issues") - though it parcels up the information into a neat format that's easily understood - treats each bug/defect as an entity in its own right, rather than as the intrinsic part of a system that it always is.