I thought this would be the case too, but its not.
If it had a walking bar type suspension, both axles would carry the same weight no matter the tilt of the trailer. But since the front axle leaf is hard mounted to a hanger, anytime you raise the level of the tongue, you take weight off the front axle.
Go give it a try...jack your tandem axle trailer up past level so the front is higher and see if you can lift the tongue by hand....now take the weight off the jack, so the front is lower and see if you can lift it by hand.
With mine, that I built here...
http://www.arboristsite.com/showthread.php?t=44000&highlight=trailer
I can take all the weight off the jack, fold it up so the tongue is hanging and lift it up almost 10". If I jack the tongue up to the same height, I cant lift it at all.
Also...Ive changed front axle tires using the jack to lift the front axle off the ground. That means all the weight is on the rear axle. Which means as the tongue goes past level towards being higher than the rear, the rear axle carries more weight.
I have noticed if I hit the manual override hard, it will tilt the equalizer towards the front of the trailer before the wheels lock up. However, the smoothness of the P3 controller, this never happens under normal or even hard braking conditions. Ive got it set at level 6.5 with the brakes freshly adjusted.