Im not sure about the mazatrol programming. Are you using tool nose radius compensation? If you are, Move the g42 to a point after the initial face cut. Here is a fanuc example.
T0101 G97 S3000 M03
G0 X(stock diameter + .1) Z0 M08
G01 X0 F.005
G0 G42 (activate tool nose comp) X(diameter you want to turn to) Z.1
G01 Z-(depth you want to go) F.003
If you are using a .031 radius insert and you activate the tool nose comp prior to the face cut you will end up being .031 off on the Z.
May be something for you to look at anyway.