Oceancrosser and ACMS, thanks too for your input. You are absolutely right, it is a minor imbalance and it doesn't 'concern' me, hence the way I phrased the initial post.
However, in 5 years of flying the same aircraft I have not seen a no.2 main tank low by 200+ kgs on a short flight. I have flown many 4-5 hour flights in the 737-800 and rarely seen 100 kgs difference, and normally there's an obvious reason.
When combined with the phantom fuel in the centre tank and an equal fuel burn from the engines I was curious as to whether the two 'unusual' occurrences on this flight were possibly related.
Thanks Skyjob and vapilot2004 for your thoughts. You might be on to something. Next time I fly that aircraft I'll check the tech log and see if it's got a history of similar events, but I won't be losing any sleep over it.
