This seems related: https://answers.atlassian.com/questions/148713/sourcetree-commit-author-change-update
I'm using SourceTree with a few of my projects, and so far I've really enjoyed it. However, one issue that seems to keep coming back up is that SourceTree seems to have its own rules when it comes to what username to pick as the author when creating commits. Specifically, in a Mercurial repostiroy, it should be using the repo-local `.hg/hgrc` to determine username. If not set there, then it should be using global settings as defined in `~/.hgrc` (in POSIX systems). Both these rules can be overridden directly in SourceTree (although why anyone would want to do that is beyond me).
It doesn't seem to be following these rules. I have a repository in which there is no username set, and I have SourceTree configured to use global settings, but the wrong username is still chosen. See here:

Redacted contents of my `~/.hgrc`:

Also, I do not allow SourceTree to modify any settings:

SourceTree *seems* to - absent any other guidance - want to use "Full Name <email@mydomain.com>" as the author rather than what's configured in my global `~/.hgrc`, and I don't get where that's coming from at all.