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"Why is this so?" - I don't know. I wish all eyepieces had smooth barrels and none had the "safety undercut". I don't think there is a standard for the distance from the top of the focusser where the eyepiece sits, to the clamp device, be it a compression ring or set screw(s)
I'd always prefer the compression ring style of holder to one with one or two set screws.
Perfect is a self-centring adapter (SCA) as used in the Hotech laser collimator and their superb camera mounting tube. Search out threads on the Hotech collimator here and on Cloudy Nights and read pages of arguments about eyepieces and set screws - eg. why bother collimating with a SCA collimator when the eyepiece is going to be pushed to one side by the set screw anyway. See also thoughts on Hotech's website.
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