The G92 G-code can mean different things depending on whether you are using a CNC mill, or a 3D printer.
This is because when the Reprap project chose it's Gcodes, and what they are supposed to do, they completely ignored the existing G-code standard, and messed everything up.
Not our fault !
On a Reprap, the G92 Gcode means "change the position of the origin (0,0,0) for this machine to a specified point relative to the current position".
On a CNC mill ( traditionally ), the G92 G-code means "set an offset for all coordinate systems".
In the Smoothie firmware, if you set grbl_mode to true, it will have the "CNC mill" meaning, and if you set it to false ( or do not set it at all ), it will have the Reprap meaning. See grbl-mode.
To see information on the G92 Gcode for CNC mills, see G92-cnc
To see information on the G92 Gcode for Reprap machines, see G92-reprap