Goldenrivett,
Using one book and working the numbers each pilot individually is different from using one laptop (or EFB) and have one pilot glance over the inputs from the other pilot.
With a book, it is filled with numbers, but each pilot on his own finds the correct line with results. The results of the second man are compared with the results from the first man.
With one laptop, the second pilot starts with the numbers that were filled in by the first pilot. It is all too easy to NOT SEE that certain filled in numbers may be incorrect.
The correct way is to fill in your numbers on your laptop and only communicate the results. The results should be identical, if not, then investigate what caused the difference. Exactly as Capn Bloggs described.
As you noted correctly, the anomaly was not a glitch in the software, as in the way it calculated, it was "just" the fact that inadvertant multiple contacts with the touchscreen could change settings without the pilot being aware of the changes occurring.