there are 4 reasons for this:
1) you have too close many points in the circle and that can canculate different arc
2)you are close to a singularity and the robot makes an akward move. try to change the angle in z axis
3)TCP as mentioned from samaniego721
4) Parameter. you need to change a parameter if you see the effects that 95devils describes
Sorry for going off-topic but i have a question about point 4. What parameter is this because I currently have a issue that the tool won't stay perpendicular to the part.
Thanks a lot.