Because it didn't seem to behave logically to me I decided to experiment. So I created a blank page with 2 rows.
The first had 12 spans and the second 10 spans with a push of 1 span.
I added a paragraph to both.

I then preview in my browser and stretch the window.
It gets so far then a limit is reached for the width of the text despite one being (in my understanding) 100% of the width of the screen. At this limit the borders just get bigger giving more space at the sides.

No my (obviously erroneous) understanding was that each span would take 1/12th of the width of the screen and scale thus - however, clearly there is somewhere a maximum at which the spans no longer expand - any idea where this is set?

Can you attach your project file Steve so we can see what you are doing?

Also, when asking for help, ALWAYS include a project file, screenshots and even upload it to the web someplace so we can see what is going on. Doing that makes getting your questions answered much, much faster. Help us help you...

Test project attached - sorry had to zip it because it refused to upload as it said it wasn't an rsd file!

This is because you have a max-width set on the rows of 1200px. Once the browser hits this point, it simply keeps the rows centered on the page. If you set the max-width to none, you will get the effect you are after.

Marvellous - wonder how that got there...

Thanks.