That's reasonable. To blame the test instead of device. Or blame the guy providing the test. This square thing is all in here, some pages ago and I'm quite sure of my estimation except when using a primary of 460mm fl instead of 500, do I loose 40 or 80 mm Backfocus? And to extimate the effect on sa. But that much is sure by (rough) measurement: Any mm distance shift takes roundabout 15 mm backfocus, if it is square f2 or f1xf2 I don't care. And that makes it easy to compensate a backfocus loss by a shorter primary.
But Whow! quite a lot. 50mm distance shift and the scope still works, good news again, that's what I hoped for. And 500+ magification at which you really are watching objects! Not too bad for a reflecting scope! I was watching 300x maximum through my 5 inch Mak but just to better separate stars. At any imaging object it's performance ends at about 250x.