Let me tell you this straight up. The default rendering engine on the WebBrowser control is fixed to ensure compatibility across all platforms.
Basically, if your installed browser is IE 7 - IE 9, then the rendering engine used is IE 7.0 only (by default).
If, however, your installed IE version is IE 6 or below, then the rendering engine used is IE 4.0, unless of course you set it otherwise.
There is a misconception that WebBrowser control uses whatever is currently installed (current IE version) but this is not true, since they do this to reduce backward compatibility issues. You can see (as proof) that this really is your problem by going to http://www.whatsmyuseragent.com in your normal browser, and then going to that website again in your WebBrowser control, you will see that it says MSIE 7.0 :).
I suspect your users (and you) who are on IE 6.0 would be experiencing the problems you've alluded to since their rendering engine being used is IE 4.0 (and IE 7 if they are running anything between IE 7 to IE9). This is really bad, especially if they are on IE 6, you should ask them to upgrade their browser if they are IE 6, then, IE 7.0 will be used regardless.
I'm not sure if, when IE 10.0 comes out, the WebBrowser control's default engine will go to 10.0 (if 10.0 is installed on system of course) or if it will remain as IE 7.0 even after IE 10.0 is released, we will have to see once it comes out. However, if we guess based on previous experience, I suspect IE 10.0 will be the last version that uses IE 7.0 rendering engine, and when IE 11 comes out, the default rendering engine will remain as IE 11 for all versions of IE between IE 11 & IE 14. This is just a guess by extrapolating what they've done in the past.
You can set the WebBrowser control to use the current installed version of Internet Explorer; if you want more information on how to do this, or how to further fix the rendering engine, let me know and I'll help you out with it.