It's the air movement.
This may be just around here but the Fire Marshall explained it in a class many years ago that the distance was because of the air movement involved with a forced air HVAC unit. To close to the supply and and it blows the smoke away from the detector, to close to the return it can suck the smoke into the unit before setting off the alarm. Either way it would take more smoke build up to activate the detector than if it was away from the vents.
Originally Posted by brantmacga
" When it seems the world is out to get you, remember, it takes 43 muscles to frown, 17 to smile, but only three to pull the trigger of a fine sniping rifle"