A better way to explain this is to note that at some point late in mission planning, after testing was complete, management decided to change the landing site.

A better way to explain this is to note that at some point late in mission planning, after testing was complete, management decided to change the landing site. Did management allow the engineering team to re-run the tests to ensure the new landing site would work? No the fuck they did not. So when the lander saw the altitude data suddenly change by 5km, the computer decided the sensor was wrong and ignored it. Based on where the computer thought the land was, with perfect precision lacking the the altitude data, it put the spacecraft down perfectly. Unfortunately, managers forgot to tell the lander that it's landing site was 5km lower than it had been programmed to expect.