It's very frustrating when the math does not add up. I have posted before about this issue with dual insurance; this is the first time I have seen it when the patient has only one insurance carrier. Whatever the cause, the math should always be correct.
I very strongly agree that the calculation should be accurate providing all info is in the table or code specific. Experienced similar issue yesterday that involved frequencies. I was told that all frequencies should be removed from all plans to allow for proper calculations.
I encourage you to view the post on Allowed amount vs Approved amount calculations.