Why is NonMusicalPaperColumn.line-break-system-details.extra-offset "upside down"?

[Kieren MacMillan]

Hi all,

(Happy New Year!)

Is there a logical reason that for most tweaks negative extra-offset values 
move the associated grob vertically lower on the page, but with 
NonMusicalPaperColumn.line-break-system-details it moves the associated system 
*higher* on the page?

I offer it would be better if extra-offset had a consistent behaviour/effect.

Thanks,
Kieren.