From this list of Chrome versions [1], it looks like WebKit 537.36 was the last version used before Google forked it to Blink, so I imagine they keep that around in the UA for backwards compatibility with sites that were looking for it.

You can even see it will stick around in the plans [2] they have to reduce the information contained in Chrome's UA

[1] https://en.wikipedia.org/wiki/Google_Chrome_version_history (scroll down to v27.0.1453 on 2013-05-21)

[2] https://www.chromium.org/updates/ua-reduction/