You guys are thinking about it too hard.
This doesn't work, and here's what is happening:
You are entering the match while your team is on offense (or defense, but bear with me) and getting 3 quests (Victory, Slay them All, Defend the Siege). You plow through and win WG, complete all 3 weekly quests.
You probably don't bother returning to WG until several days later, giving the illusion that the quests have had time enough to reset due to some server error, etc. Instead, what happens is,
you return while you are defending and magically, there are several quests to complete! The "Slay them All" quest while defending is different than the one while assaulting, at least to the server. The siege quest is completely different, with a different name. You also now have Toppling the Towers.
It is much more likely and plausible that everything is working as intended, only there are separate quests for defending and separate quests for assaulting.
Sadly, you aren't doing the same quests twice.