I wondered that too, but its quite unusual to suddenly have a large fuel imbalance. Yes, it could have been a computer error, but even so, I still think I would have thought "fuel" looked at the fuel pages, and added up the total fuel on board.
I know its much easier with the hindsight, but surely this is what the Captain in particular is paid for? We don't have flight engineers on board, the pilots are meant to be doing his job - monitoring the systems. The fact they already had an abnormal oil indication on the same engine should have at least led them into some close checking and monitoring.
We often get an Air Bleed problem on departure, which is usually spurious, but will still check and double check, and then check on it again a bit later on. We don't assume its the same spurious reading.
Interesting that the pilots did not want to be interviewed for the programme.
It had some similarities to the crew who flew to Vienna with gear stuck down and didn't monitor the fuel they were using - again, only just made it to an airport. They had other options but it came down to the basics - monitoring.