why the axis synchronized not accurately

when the 3 axis are synchronized to 1 virture axis with a fixed setting velocity value,my problem isas below :
I traced each axis speed curve,found that wave fluctuation is under 0.1/60 Degree after my optimization work,but the tension between each axis(paper) are not stable, why the axis not synchronized accurately and dynamicly? How can i improve the bad effect? give me the specific parameter.

who can give me the proper answer.Thanks a lot!