Programmers use many different buckets to hold values in. Over years as programs get modified, sometimes new buckets are made to hold similar but not quite the same info and a programmer incorrectly grabs the wrong field to use in the various calculations. Some times, the programmers have no idea what the information they are putting on the screen really means.
I'd just put it up to poor Q&A which actually is quite common from things I've been involved with. Sometimes it's a choice of economy because the changes required might actually mean the whole thing has to be tested again as they might break something else or just be too time consuming to re-test.
I once was part of a project that never went live to my knowledge because although the product we made worked perfectly to the best of our knowledge the company that was created for, never could test it completely before the next software release of the OS, databases, or other products used for the entire product. And this was just a IVR that you call in and talked too. But the complexities on the backend the user couldn't see were enormous.