I understand and accept 'the science' behind the so-called 'eyebox', it makes perfect sense. However, I have never really fathomed out why some binoculars have a reputation for being particularly finicky or unfriendly, and others of the same objective/magnification formula (8x42, for example), or even an entire model line at the various magnifications in the range, do not. The knowledgeable ones will usually maintain that it is the operator of the binocular which is the problem, not the binocular itself, insisting that it is simply a matter of setting up the binocular correctly. However, those same people also maintain that it is important to try before you buy, to make sure the binocular fits (or works for) you, eye relief and IPD allowing.
I get the theory, I don't really understand why the reality appears to be somewhat different.
I have experience of a pretty broad range of binoculars, and some I'm able to just pick up and use without thinking about them, others (despite adequate eye relief and IPD range) I simply can't, no matter how hard I try (including ages spent fiddling around with o-rings). This really doesn't bother me, apart from the fact that it goes against 'the science'.