I understand, but the wide blade doesn't go to the chassis. Can you see the legend on the left there where the chassis ground symbol is different than the B- ? The center prong on the plug...the widest and longest center one in the picture...does not represent the prong in the grounded circuit conductor (often technically incorrectly referred to as "neutral" in a 120V single phase circuit). The prong to the chassis is the system ground. The metal chassis is at the same potential as the earthed grounding conductor of the building wiring.
Scroll down and see the right hand column:
http://www.electronic-symbols.com/el...rs-symbols.htm