Is this true? I've never seen this mentioned anywhere, and some googling doesn't show me any mention of this. Do you have a reference to something that verifies this and/or explains it?

I can totally understand how CSS expressions would be re-evaluated due to a repaint (resizing the window, manipulating DOM elements with javascript, etc)... but it doesn't make any sense that simply moving your mouse would cause this.

It's a simple optimization to only reevaluate a formula when the constituent variables change (in these cases, the viewport sizes and root font size). I know that at least Qt Quick implements this optimization, and I would assume that browser developers already do that, too.