My Rotary Tool on my 50watt SFX Laser Engraver is cutting circles in oval shape. I can change shape by changing work piece diameter but its never the same

I can change shape by changing work piece diameter but its never the same.I can change it to half the actual diameter of a cup in my rotary settings and get it close but its always skewed on one side or the other. My setting all look rights. I need help

Which type of rotary is this?

I have two, a chuck type and a roller type.

For the roller type, if I remember correctly, you set the mm per rotation value to 360 and set the roller diameter to whatever it is on the machine. As far as I know you use these values from here on out.

For the chuck type you set it to the MFG recommended value, which is 160mm on mine, and then set the circumference to the actual size of the cup (diameter will be calculated automatically or vise versa). You’ll have to set this each time if you engrave on different sized cups.

My machine says my rotary tool should be set to 6400 steps per rotation. I can do the test and cup will rotate 360 degrees forward then back 360 degrees to starting point like its suppose to but when imake a burn it burns in an oval shape. I can cut the actual number in half on diameter of cup and it will burn closer to a circle but it never right shorter on one side or the other. I used this rotary no problems for last couple of months and few days ago it started this, driving me crazy,

My Lightburn set up says "steps per rotation}, not “mm per rotation”