Kind of a bug in fact, but if you have HTML added above the doctype (as with what Google cache likes to do sometimes) the document gets thrown into "quirks mode". And when in quirks mode, regardless of the browser, classes are case insensitive. So having .Sidebar on the html tag actually will render the whole document 250px wide floated to the right, etc. as you would expect out of .sidebar
Here is some more information provided by @kylerc on this: https://xenforo.com/community/threads/ui-x-paid.57944/page-275#post-998017
A suggestion would be to change it to .pageHasSidebar for example.