Is Defra trying to screw the DfT?

This morning Defra published a compilation of airport noise charts, showing the area around airports, and the various decibel bands.

Nothing odd about that, until you look closer. While the DfT tries to avoid showing an area greater than its preferred 57db limit (and rejected the recent ANASE report which found that people are disturbed at 50db), the Defra day chart goes down to 55db, and the night one to 50db.

Could those crafty civil servants at Defra be trying to undermine their transport colleagues determination to ignore the ANASE report's findings? Why else would they show a 50db limit if, as the DfT believes, people are only affected from 57db and above? Or is it just early and I'm being conspiracy minded? Answers on a postcard to: I don't like aircraft noise, c/o the DfT, 76 Great Marsham St, SW1P 4DR...