The idea is to catch them early by simulating future versions now, which in their belief is best done by perpetuating traffic that contains bullshit versions on the wire. It's fairly clever, in that any implementer who doesn't want to break significant portions of web traffic now must code their implementation in a way that anticipates Chrome's future versions, which is most sanely done by coding their implementation in a way that happens to match what's in the spec. Meanwhile, noncompliant implementations will likely break for a huge amount of users, causing widespread uproar and, presumably, pressure to fix the problem.
Google's clients (e.g. Chrome, Android) and servers originate and receive a high enough portion of traffic that they could pull this off on their own without anyone else's buy-in, and still achieve the intended effect, but they're reaching out to the community to build consensus. It's a hacky idea, but one that backs noncompliant implementations into a corner now, instead of them sticking around forever and causing incompatible changes later down the road.