Eye relief published, even if accurate, is just one factor. If the eye lens is recessed, eyecup lowest setting too high, &/or other ergonomic reasons, the usable eye relief can be reduced enough. 20mm = 2cms, not a lot even if the full amount is usable.
Then there is exit pupil attainment, retention and behaviour. I found Delites good here, but not Delos or T4 Naglers. TV 40mm plossl good, less so their 32mm.
Different FL eps in a range can have good and bad exit pupil behaviour. Pentax XW 10mm good, 7mm downwards less so. Pentax XL 5.2mm better than XW 5mm.
Some eyepiece ranges eye relief shortens (too much) but their exit pupil behaviour is better. Comparing Naglers 13mm T1 & 12mm T2 was an example.
So it's not just eye relief on its own, but how much work an eye does not have to do, how much an eyepiece becomes a natural extension of an observer's eye, not a challenge to it that counts. Like wearing glasses, whether prescription lenses or just shades. We are not in the forces having to adapt as best possible to designated equipment, however good. Designers making good optics should be designing both comfort and performance with equal priority.