There doesn't seem to be any complete explanation, only a mention of it in the "How To Handle Multiple Committers" page:
If you are using a version of Mercurial older than 1.0, it's important that each user's umask be set to 002 or something similar. If it's set to 022, group write privileges will be masked off for files that users create, causing other users to be unable to modify them.
You can change the default umask for a user by editing their ~/.profile and adding the line...
umask 002
Alternatively, you can edit /etc/profile to change the system-wide settings.
  Changing the /etc/profile will not work if people check in stuff with the hg command, as that file isn't read by non-interactive shells, /etc/bash.bashrc is a better place to put the system-wide settings.
Otherwise since Mercurial 1.0, the default permission is inherited from .hg/store.