The Construction Grid can actually be referenced from wherever you like, all you need to do is set the Skewed Grid, then give it an angle (if any).
The Snap Grid is always referenced from the location of the User Origin.
I never use grids, neither do I encourage their use to my clients, (I don't believe they're needed when working in ArchiCAD, and we should enjoy the freedom) but I guess there must be some benefit to referencing the Snap Grid to the User Origin as opposed to simply referencing it from the Skewed Grid's Local Origin.
In fact it seems like the relationship between the Construction Grid/ Skewed Grid/Snap Grid could be simplified, even just by making them all one in the same, but maybe I'm missing something.
Cheers,
Link.