Let me fix your Math for that Lascannon Hit:
Land Raider Penetrate (1/6) * chance to destroy on Pen (1/3) = 1/18 Chance to Destroy
Rhino Penetrate (2/3) * 1/3 chance to destroy on Pen (1/3) = 2/9 (or 4/18) Chance to Destroy
Amazing! A Lascannon is 4 times more likely to destroy a Rhino then a Land Raider.