Well, about 12% of Chrome page views are from old versions so "almost no users" is a bit of a myth or exaggeration. But it's definitely a smaller proportion than other browsers.
Firefox also auto-updates, but has a longer tail of users from old versions since it's been around longer and had a more obtrusive update process until recently. Currently about 32% of Firefox page views come from old versions, but this is decreasing steadily now that Firefox 3.6 users are being auto-updated to the current release channel, and now that updates are more "silent."
Once SPDYv3 is enabled in a release channel the Chrome installations which only support an outdated SPDY version will rise suddenly if the current update trend is not reversed in a future version.
