For the "why", you'll have to ask the guys who designed the trans...I can only speculate.

The deeper channel feeding the rear servo would make for both a quicker apply and a quicker release. This would seem to be beneficial but the downside is, when the rear band is applied in Reverse, the in rush of higher pressure fluid can cause band/piston breakage. Line pressure in Reverse is increased by a factor of about three and the cold pressure can exceed 300 psi.

In later years they addressed this problem by using a restrictor orifice and later yet a check ball to route the incoming fluid through a smaller orifice and outgoing fluid through a larger orifice.