The WHATWG spec is a future-facing document; lots of ideas are incubated there. The W3C spec is a snapshot of what works interoperably – authors who don’t care much about what may or may not be round the corner, but who need solid advice on what works now may find this spec easier to use.
I was honestly unfamiliar with the WHATWG spec and now I find it super interesting to know there are two working groups pushing HTML forward in distinct but (somewhat) cooperative ways.
Kudos to you, Bruce! And, yes, Vive open standards!
Source: CSS Tricks Editing the W3C HTML5 spec