Why does playing xerj and succesfully attacking someone give you only half of his available resources?

Most recent example: i attack someone having 200 000 vespene, i win the battle and have 30 overlords with my fleet. Now that should accomodate 15 000 x 30 = 450 000 transport capacity, right?

But i only get 100 000 after the battle, i send a spy probe immediately after and see the guy still has 100 000 despite me winning the battle and having plenty of space to loot all the vespene.

Same situation has happened every single time i launched a succesfull attack since switching to play xerj. Is there a reason for this or is it a bug?

i am terran and have the same issue...i loot 50% of resources whether my target is terran or zerg....as long as my target is active, i loot 50%..inactives 90%