In IE11, checkboxes are rendered as a flat style with a black checkmark, as apposed to the 3D look rendered by IE9. When using a RadTreeView with checkboxs and form decorator, the checkboxes are not styled to look like the 3D ones. Checkboxes that are not in trees are styled, and therefore, the checkboxes look differently. It's expecially apparent when both treed and non-tree checkboxes are on the same page. See my support ticket 815961 for my question about this.