I finally understood why I always have a difference between the seconds in my INS and those on the map... The reason is simple but I did not pay attention:
Seconds in mirage 2000's INS are not seconds but a percentage of a minute !
For example, if my plane is on parking with the map (or F2 view) showing this coordinates: N42:52:12 - E041:07:28
So, the correct coordinates to enter inside INS are: N42:52:20 - E041:07:47
(12x100)/60 = 20 for the North (12 is 20% of a minute)
(28x100)/60 = 46.6 = 47 for the East (28 is 47% of a minute)
This is why we can enter until 99 in seconds for 99% !
If you don't want to do the math between the coordinates on the map and the coodinates to enter, you can see the corrects ones on the kneeboard under 'initial position".
If it can help someone... and sorry for english !