The short answer is it is the geometry that causes the inside wheel to turn a little further than the outside wheel, which gives you good control on turns.
When you put the arms in front, it does just the opposite, and the tires sort of skid through the turns

Because T buckets traditionally have no room for the arms to go behind the axle, because of the springs and perch occupying this space, the arms have to go in front. I read where Total Performance arms are engineered to correct this problem, but I can't see how, looking at the set we used.

What dictates if you have correct ackerman is if you drew a line from the center of your rear axle housing to the center of your spindle kingpin, it should pass through the center hole of the arm, where the tierod bolts on.

Perhaps one of our computer experts can post a drawing of this, as they so often do, to help understand the principle.

Don