Ok, I thought it had something to do with my custom.css but it hasn't. I checked on a fresh installation.
Steps to reproduce the issue:
1. update masthead to 1.1.3 on a quickstart fresh installation of the template.
2. compare this page on the demo site currently online with the same page on the quickstart installation of the template : you'll see the masthead title should be aligned to the left but it's more to the center than it should be.
LASTLY I 'm still interested in knowing how to align the masthead title to the left or to center, as I said in my first post, regardless of the issue I've just reported.
Thanks.