Dear Mark
Splitting is done in case we do activity independent planning at KP06 level. Seems that at KP06 you have done activity independent planning (without reference to the activity) and later you have done splitting of planned Cost elements to various or one activity.
System consider expenses planned in the activity independent planning as Fixed cost. Hence this cost is considered as fixed cost and not the variable cost by the system.
Regards
Rajneesh Saxena