I suspect that the Super 40 history has something to do with this (30 x 173 necked-out to 40 mm). When first developed, the APFSDS round had a very long case (218 mm, as I mentioned elsewhere) for a puller-type sabot, with a shorter (165 mm) case for the HE loading. This didn’t work too well, so the eventual length chosen was 180 mm, with the APFSDS having more of a mid-length sabot design.
The 50 x 330 was also designed for a puller sabot, with very little of the projectile exposed above the case. Maybe the US decided to adopt the mid-length sabot design they were familiar with, which meant a shorter case. Otherwise, the HE round would be mostly buried in the case with the rotating band high on the nose - possibly that would cause problems?