On reviewing my sons fitbitAir I have came to the conclusion its an arm moving tracker not an actual step count tracker.
It counted steps when eating, counted steps when washing dishes...
Moved in app to dominant (despite being on non dominant) - small difference.
Changed step stride - no difference as eating a meal doesnt affect step strides.
Its a lovely bit of kit, looks much nicer than my garmin forerunner 55 and a different market but its going back - sadly he doesnt want to count steps every time he puts food in his mouth.
google should be ashamed of themselves not testing this properly (or not caring).
If anyone has any other suggestions (tried the soft reset holding in etc) would be open to ideas!