I found the problem.
The problem is that in order for my computer to have a normal aspect when my monitor is plugged in, it is set to display text and UI at 125% which is the recommanded setting.
Then, if I turn on my computer with the monitor already plugged in, The dimentions are correct (1920×1080)
but if I turn on my computer with the monitor not plugged in, and then I plug it in, the dimentions are not correct (2400x 1350)
This explain how to get correct data, this problem should still be fixed my tobii developpers