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Optical design...telescopes inherently have field curvature and it depends on design, focal length, etc as to how much. This can cause the image to be out of focus at the edges while in focus in the centre, i.e. the focal plane is not flat.
From what I understand (not much!) - and there are experts here that know way more about this than I - eyepieces can also exhibit field curvature and the combination of that from the scope and the eyepiece can either cancel each other out or make it worse, depending on design parameters. SCTs typically have a fair bit of field curvature so it's speculated that the eyepieces Celestron supply have taken that into consideration by design.
Needless to say, this is probably further complicated - or maybe not - for the Edge HD scopes, which have the correcting lenses in the baffle tube to help give them a flatter field i.e. the focal plane is less strongly curved so that the edges and centre are closer to or are at focus at the same point.
I've oversimplified here and I'm sure there are other factors too...
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