Wrong spark plugs?
 

Last week I bought a used 2009 Civic GX (natural gas vehicle model) from a private party who was the original owner, with 114K miles on it. I noticed recently that the engine vibrates more than usual, and the engine shudders when I come to a stop at a red light. It is more apparent when the AC is turn on.

Anyways, yesterday I decided to do an oil change, ATF drain and spark plug change. I bought the NGK Laser Iridium IZFR7F-8DS for the 09 Civic GX model. When I took out the old plugs, I noticed the model number on the plugs say IZFR6K-11S, which are actually for the Civic DX gas model, (according to NGK's website). Story short, the new plugs eliminated the engine shaking, and I now experience no rough idle.

I don't think the previous owner had the plugs changed because the plugs look really worn out, and besides, the scheduled service is 110K for plugs. She took it to the dealer for just about everything, including routine oil changes and tires.

It is possible for Honda to install the wrong plugs when car was being built?