Relevant cost Ch09 – second video from the lectures

Dear Tutor,
In example 2 John brilliantly points out the extra costs note by note.
Regarding machine OH (note 5), something bothers me:

If we recognise 6,000 hours of electricity cost for the extra job, why don’t we recognise 2,000 hours saved from the normal job since those hours were not used? That would reduce our costs by 6,000.
Thanks!