I had to hardcode some values for vertical "split" to pre-assign values that are basically necessary for the vertical split to look nice. I can't remember all the details, but for example you can forget about portrait images displaying entirely if using vertical split in wide mode. And text becomes too wide to read ... As you may know, text width should be limited at book-page width for optimal reading.
There may have been a few other things also. In conclusion, I hardcode a few settings so that the option becomes useful for most X3 users and to protects again usability issues. I really can't see any functional purpose of having the vertical-split layout set to wide.