The chord formula is 135 (That's the 1st 3rd and 5th notes of the scale : C,E,G) - What PG plays for the C major chord is : C, E, G, C, E - 1,3,5,1,3 - It's still using the same chord formula/harmony because he hasn't added any new notes, just using 1 3 5. That's why it's still a C major chord...