Different Spark Plugs for Driver/Passenger Side?

I'm just over 30,000 miles, and changed the spark plugs today. I purchased the Denso plugs that the dealership recommended. I changed the driverside plugs first, and the ones that I removed were NGK... Then, I changed the passenger side plugs, and to my surprise, the ones I removed were Denso just like the ones I purchased for the change... I bought this truck new with 6 miles on it... Why would there be 2 different brand plugs on the same engine?

~Troy

 

The heck with every 2nd Gen V6 owner...you'll find this in every V6 in every model as far back as the mid-90's (or earlier?). My Dad was a Toyota mechanic (retired) and it was this way on every V6. Don't ask why because you'll get lots of guesses.