Gap makes all the difference in the world.
If the plugs were all that far out , chances are the mechanic that put them in either didnt bother gapping them at all ( you know" Billy-Joe-Jim-Bob told me I don't need to gap 'em, the'll work gest fine out the box"), or gaped them the same as all the Chevys he worked on that day.

Some gap erosion is normal over time , but to get three times the spec, it most likely was put in wrong in the first place.

Just because a shop has been around a while , doesn't mean they know what they are doing. The local independant Volvo shop around here has all of its customers cars wheel bearings so loose the tires are practically flapping in the breeze.