What's the difference between Mercurial's "tip" tag and the "default" branch
tip can be on a different branch than default.
hg uses the latest revision your "-r somewhat" matches. if somewhat is a branch, it updates to its latest head. if somewhat is a tag it updates to the revision this tag is associated with.
Tip is the most recent changeset in the repository. Default is a branch. Tip is a special Tag (a named changeset) which always refers to the most recently changed Head.