After the CNC cut, there is a problem like I attached a photo below..

when first router cutting,  it cut around 0.5mm thick more..

and then after the second router cutting, it cut the correct dimension.. so I can see 0.5mm thickness left on the bottom of the board.. 

Is this the problem regarding the [ 0.5 thickness left for first nest pass ] ? please refer to the attached photo.

If not, Do you know why this cut problem happened?