If the car had been well cared for (which lots of early Z28's were ; they are like Corvettes, they were/are either driven very hard, or babied!)..
; anyway, if they were well cared for by a 'car nut', it is possible that the owner removed the plug to ensure there was no trash trapped (which would cause rust with moisture), and to have cleaned the plugs prior to re-installing... and Yes, I've done things like that before..
It's also possible that the undercoating wasn't done consistently by every person who ever did the job.... (although the undercoating on all my fenderwells seems to be very consistent and per a precise pattern....
Gary