If you have the cutter offsets "ON" then Mach creats a "barrier" round the tool, equivelent to the diameter of the tool. If you are cutting a circle, this is apparent, becasue the tool is on the inside, (or the outside) of the circle, so the path the tool decribes is not the diameter you asked for, but is the radius minus (or plus on the outside) the tool diameter.
However, it is clear, on a circle, that this barrier must ne all the way round the tool, since at some time or another, all the tool comes into contact with the work.
When the circle is complete - say you have asked it to go all round from X0 to X0, then it does, buts stops short of X0 by the tool diameter when it arrives.
You can either describe the circle by putting the radius described by the tool centre (and doing away with the tool diameter offset), or adding a further line to make the tool go over the X0 line by adding in an amount equal to just over the tool diameter.