Switch units, mils to mm, inaccuracies

Hello,

 I've a pcb board with routing completed in Orcad PCB 17.2

Rules are: trace with 8mils, trace clearances 8mils.

I routed the board in mils and all sounds good.

Now, I've to do some change in mechanical quote (board and hole places)...but...if I switch to mm and back to mils....there are a lot of inaccuracies error...like "Clearences are 7.999mils and not 8mils"

How to solve? In Mentor PADS and in Altium Designer there aren't these problems.

Thank you

Stefano