This is from the manual:
Moving and Re-sizing Frames using Arrow Keys The following are for points, millimeters,
picas*, centimeters, ciceros
Arrow keys Move item 1 unit
Ctrl+Arrow Move item 10 units
Shift+Arrow Move item 0.1 units
Ctrl+Shift+Arrow Move item 0.01 units
* Movement units for picas are points
The following are for inchesArrow keys Move item 0.139 units**
Ctrl+Arrow Move item 1 unit
Shift+Arrow Move item 0.1 units
Ctrl+Shift+Arrow Move item 0.01 units
** This is about 10 points
Now, I use inches, and I did some testing. The arrow key movement was not 0.139, but 0.0139. I also found that with snap to grid off, the arrow key would stop just above, and just below my 1/2" border guide, but with snap on, it would snap to the guide. Right at .500.
All that being said, nowhere in the manual does it give any instruction on changing the settings.
[edit] I just went back to Scribus, and using the keyboard did NOT snap to the guide. Hmmm.
P.S. - In Indesign they call using the arrow keys 'nudging' and it can be customized:
https://smallbusiness.chron.com/nudge-indesign-44317.html