[CM30]

The W3C prefer their specifications to be 'finished' at a certain point, and the WHATWG simply add their changes to the living spec.

So for the most part, changes get made in the WHATWG spec, they get copied to the W3C one, the W3C then stops copying additions at a certain point (say, when HTML 5.0 is considered 'complete') and then moves onto version 5.1. So version 5.1 is basically all the WHATWG additions that didn't make it into 5.0 before the spec was 'finalised', along with various changes made because the two groups don't get along with each other too well.