It seems that there is a discrepancy between these two workflows:

Workflow A:
1. $ git cl upload
2. $ git cl web
3. Click "Publish+Mail Comments" in the review tool.
=> Mail is sent to reviewers and Cced emails (watchlists).

Workflow B:
1. $ git cl upload -s
=> Mail is sent only to reviewers that are in the R or TBR sections of the CL description. Cced emails (watchlists) are bypassed.

(Somewhat related, I filed  issue 610992  for the possibility to add a custom message to the email that is sent.)