Fred,
G28 X ..... Z..... axis will move via an the intermediate point. When you don't specify an intermediate point by just coding
G28 the current point is considered the intermediate point. See 10.7.8 in Using MACH3TURN manual for definition of the
g codes ( which can be confusing ).
I strongly suggest you actually spend some time playing with the command using singular axis and combination of both axes when using G28. The actual axes movement can get confusing quickly so be care full.
RICH