Brundle thought like the rest of us were surprised that Button's time could come from running a lot of fuel. I see nothing wrong, all previous experience would suggest it was unlikely that it could be that fast with five extra laps.
As long an Brundle keeps calling it sensibly I'll be happy, better than inventing something that is unlikely just to make it more interesting (naming no names
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Brundle had already 'backtracked' by Sunday grid walk. He said something like 'watch this man' in reference to Button's fuel just after interviewing him on the grid. I think by then Martin had heard a whisper.