Whattaya think @Rudy ? Phone shit that I could conceivably change if I were anal enough to be bothered by it?
In my case, if I increase the text size in the phone's settings (which affects everything on the phone), that has occasionally thrown off the layout of some sites.
It's hard to say what it might exactly be, as there can be other variables that might cause a shift in the layout, like the "dpi" setting of the phone (dots per inch...basically, the number of pixels per inch on a display, which is one of those settings buried deep in the phone OS which may not even be accessible without connecting to the phone with a USB cable and issuing commands from an attached computer, or downloading a specialized app which can change it).
Even a change or update of the forum software could cause it (which is one thing I'm looking hard at on another XenForo-powered site I'm in the middle of developing). But since the current version here hasn't changed for over a year (version 2.2 has been around for a couple of years now), that's likely not the case.
This forum, BTW, can be saved to a home screen on a phone or tablet, and a version of the forum will open up in a "chromeless" version of the browser without any of the browser controls. That makes it work similar to an app. However, it would not affect the layout, other than providing a little more room top and bottom for scrolling.
Wish I could give you a concrete answer!