Is it due to mechanical bad accuracy of the device ? If yes, may it appear after hours of use ?
No, it is the nature of the beast.

What the Agilent component does to manage this ? Do you know the approximative cost of a such device ?
I have not used it for ages and do not recall; however 2-8 months ago the same came up in this forum and someone mentioned the part number.

Feeling nice today, I found it for you, entered by Bruce Bailey
http://literature.agilent.com/litweb/pd...5895EN.pdf

As I recall, the chip is a couple of bucks in small quantities.

Erik