USUALLY, that means that a lazy-a$$ mechanic only changed the easy access plugs last time and left the hard to change plugs for the next 'sucker'.* 3 new plugs will be enough to show some improvement for the driver and the average customer will assume that is as good as it gets. Meanwhile, the half-a$$ mech. gets paid in full for a job less than half done. (Since the other bank of plugs would have taken WAY more than half the billable time!) On transverse engines with the back bank smashed against the firewall and blocked by the intake manifold, this is VERY common.
* Also, to be fair, by the wannabe shadetree mech. owner that has not the skills, wherewithall or nuggets to do them.
