I'm sure incompetence plays a part, but two other factors at play are the fact that no one person really understands how things work, and when things start getting that distributed it's probably fairly likely that there are logical fallacies that have been built into the system and adhered to by strict processes for decades. It doesn't take much fudging for a bureaucracy to paper over these edge cases, the accounts shrug and say close enough, and the consultants are then put in an impossible scenario where different parties give them different business logic and no one has the authority or will to reconcile the issues from the top down.

Take a simple payroll system. Start with basic employees. Add in contract workers. And, this is the hard part, handle the exceptions - those who get paid more even though they are at the same level, those who get more vacation, those who have legacy benefits. Scale to an entire city.

Test. Because if people don't get their paychecks, you have a very angry group of customers.