I found the solution : Set snap size to 0, then to 1.27mm :-). Olivier

Not sure I understand the issue exactly. In your first example, you have set the snap to 1.28mm but your grid is set to 2.54mm so your tracking will be off the visible grid. Your later snap of 1.27mm is an exact subdivision of 2.54mm so track placements will lie on, or midway between, the visible grid. It also looks like the package you are routing to is on a pitch that is on a multiple of greater than 2.54mm or 1.27mm so the track end point - which will snap to the centre of the package pins - will be off the visible 2.54mm grid.

I had confused 2.54 and 2.56 :-).