Quality Digest

Can someone explain what a 6 sigma process is said to produce a 3.4 ppm, when the same process could produce 2 ppm if it was centered. I understand the 1.5 sigma shift theory, but I do not understand why we would need to state PPM as 3.4, and assume the shift. I a not "Black Belt" educated, so keep the answer "Green" please.

Thanks for your time...