Of course its misleading. The emissions tests are meant to represent emissions standards of the vehicle as a whole. So fudging the engine management to cheat it clearly is cheating.

Its a bit like putting a bung in your exhaust for a static track noise test, then removing it before going on track

Or having your head lights aligned then pushing a button that makes them dangerously dazzling once you left the MoT station

MPG isnt governed by law, emissions are. The manufacturers can get away with MPG figures because if you bother to read the small print, they will tell you EXACTLY what conditions the MPG figures relate to.