This happens because of the .pageWrapper div that has margin: auto for screens wider than 1250px, which effectively centers it. The problem with that is that it is a parent of the <form> element. The RadEditor dialogs (which are RadWindow controls) render as child elements of the <form> element (the same goes for the modal background div) and they have position: absolute. Thus, they inherit the centering from the .pageWrapper div and their offset is applied according to their parent element.
Said shortly, with this CSS, this behavior is expected.
To avoid it, you can move the pageWrapper div inside the <form> element so it can hold the content and provide centering.
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