Fuel efficiency in motorway jams

We spent hours in queues on the M6 and M5 - never really stationary but crawling in a stop-start fashion.

I had thought this was supposed to ruin the miles per gallon values. However we did 389 miles  over nearly 12 hours, mostly on motorway and with maybe 4 hours in queues. We used 24.74 litres of diesel which is 5.44 gallons. That gives us 71.5 miles per gallon. This is better than last week. One difference has been taking off the roof box - but given the queues I wasn't expecting an improvement!