Boy, even the background area clear has way too many machining marks. I tend to agree with the others. Looks like there is some backlash or general looseness in your Z stability and/or not perpendicular as well.
Do you get a smooth/flat table when you surface it or lots of machine marks between passes?
Background should be same as your table with surface cut.
I know you said the part is firmly held down otherwise I would be suspicious of movement as well. In fact I'm still suspicious it may be lifting and/or moving.