Meteor update doesn't update the meteor version used as expected on El Capitan

I got this message in my Terminal.

Meteor is available. Update this project with 'meteor update'.

So I ran meteor update in my project folder, but got:

$ meteor update
This project is already at Meteor 1.3.1, the latest release.
Your top-level dependencies are at their latest compatible versions.

Why didn’t it update my project to Meteor

Looks like I had to manually specify the version: meteor update --release