most o the time this is due to numbers. if one side is a full raid ( 2 and half full raids? ) an the other maybe one raid or less. Tenacity or not, the numbers still overwhelm the underdog side. this is not always true. I am on a server where Ally outnumber us by a ton. and the horde win with less than half the ally numbers sometimes. in fact IF we ever win it is when we are at least with 1-3 tenacity. Tenacity only heps against players, when you jump into a siege to go kill walls.. the tenacity does not affect the vehicle. So the zerg just decimates your attempts to get through the walls most the time.

So one fix that Bliz will be implementing is a one to one queue. Should go live for WG in a future patch ( I'm guessing the pre-cata event patch) Now if you still lose WG most the time, it will be based on skill, since one side will not overwhelm the other like they can today.