MRC files define an origin in the binary file header.  Sometimes that origin is the center of the box, sometimes the lower left corner, sometimes in different position.  ChimeraX uses the center defined within the MRC file.  If you open the file in ChimeraX then you can see the origin used with menu Tools / Volume Data / Map Coordinates.  The "Origin index" indicates the grid index where x,y,z = 0,0,0 (Angstrom units usually).  In the example below it lists the index as 0 (which is shorthand for 0,0,0 since it is the index in 3 dimensions), meaning for this MRC file the origin was the lower left corner of the file.  The origin index could be changed with a ChimeraX command, e.g. "volume #1 originIndex 191.5,191.5,191.5" to move x,y,z = 0,0,0 to the center of the box where this map has 384 by 384 by 384 grid points.  All ChimeraX commands like moving a model with the move command use physical coordinates (Angstroms), not grid indices.

Tom