Q: Resolutions besides "default for Retina" and "native" are low-resolution when rotated

[Cross-posting from my question in the El Cap community. See also this question in the MacBook Pro community.]

Running 10.11.2, previously 10.10.5. I have a Dell P2415Q 4K monitor connected via DisplayPort. Actually, I have two: one in landscape mode and one in portrait.

The landscape monitor is recognized as Retina-capable. In SysPrefs > Displays > Display, when I click on "Scaled" I get the nice little scaled-Retina-picker boxes (looks like 1504x846, 1920x1080, ..., 3008x1692). If I option-click on "Scaled" I get a list, including those resolutions and others, up to the native 3840x2160. So far so good.

However! For the portrait monitor, things are not so hot. Clicking on "Scaled" does not show me the nice little boxes. Instead, I have a list like this:

• 1440x2560 (low resolution)
• 1296x2304 (low resolution)
• 1080x1920
• 846x1504
• 648x1152

And, as advertised, the "low resolution" options look terrible—no Retina scaling at all. Option-clicking on "Scaled" gets me a larger list, but again, all resolutions above 1080x1920 are low-res, with the exception of native 2160x3840. I've looked at QuickRes, I've looked at SwitchResX. Nothing I've found has been able to make those higher portrait resolutions Retina-capable. So:

1. WHY? What is it about rotated Retina that OS X just can't do?
2. Does anyone know of a way to fix it?

Thanks much!