thats because they dont have thier stuff fine tuned, lots of reasons why that may happen but its normally the growers fault, i use 2x150watt lights and should get by the looks of things 3-4oz of dried herb.

a 250watt bulb has better penetration than my 150watt so if i can get 3-4oz with them you can get 5oz from a 250 with practice, ive seen a 8 ounce harvest with a 250 so its possible.