this is why:
"the disclosure of accurate engine size to the consumer. While we have chosen to use the long standing factory branding of the "80cc" name, the true displacement of the bicycle engine is actually 66cc. The debate has been open for a long time in our motorized bicycle community as to what the correct engine size is and we hope to provide the answer. The volume of a cylinder is calculated by using the formula Volume = Pi x Radius squared x Stroke (multiply 3.142 x radius of bore squared x stroke length divided by 1000). In the East, the displacement of an engine includes the area of the cylinder head's combustion chamber as well as the volume of the cylinder. In the US however, the volume of the cylinder is only considered when identifying engine displacement. With a 47mm bore X 38mm stroke this equals a displacement of 66cc,
This engine is one of the biggest bicycle engines on the market and is the same as the so called 80cc."
(quoted from pretty much every place that sells these engines...)