The figure of 768 pixels is commonly used for what is called in web design a "breakpoint", where the layout is changed from desktop to mobile format (Natda, 2013).
You can try this with the Moodle Demo using the Mobile Browser Emulator extension for Google Chrome, or simply by making the width of your web browser window smaller until the columns disappear. Try your own installation of Moodle and see if you get the same result (there are many non-responsive Moodle themes still in use).
The Mahara e-Portfolio has a similar responsive design by default, you can try this with the Mahara Demonstration.
Natda, K. V. (2013). Responsive Web Design. Eduvantage, 1(1). Retrieved from http://cms.omvvim.ac.in/index.php/eduvantage/article/viewFile/18/pdf