Barry, It is certainly possible on any particular engine for the adjuster nut or the lock nut to be messed up to where they will not hold their position...
And the FSM suggests that if they won't hold their setting that new ones should be installed....But as a generic reason why our engines normally get tighter between adjustments it is the cam/valve train design.
Looking from the top of the valve, with right hand threads.... if the adjuster won't hold I think it more likely that it will get looser by traveling towards the big end of the valve with the pounding from the cam rather than defying gravity and getting tighter.