I have written a lengthy article to explain why Masonry does not (yet?) do left-to-right ordering of content.
Note that this has nothing to do with Pinbin theme, but with the underlying Masonry javascript library made available in Wordpress.
I have successfully used a workaround suggested by others (thank you, cavoirom) to implement this Masonry feature request.
You can find the complete article here:
*Removed