The measurement of g's per watt is merely an estimation to compare efficiency of a grow.
There are millions of methods out there for growing and everyone likes to think of them selfs as the greatest grower.- Generally speaking the measurement usually refers to the wattage used in flowering.
it doesnt make sense to me with the grams per watt ratio, if you take 2 of the same plants with the same veg and flower wattage. But you veg one plant a month longer than the other then the plant vegged longer should be the superior yeilder right? So really it depends on how long you veg for with the wattage
You misunderstand its purpose.
The g's per watt is used to determine the overall efficiency of the grow. A grow op is limited by multiple factors and each grower has to find a method that best suits his room/space and wattage.
Anyone can keep a plant vegetating for 4 months. But to flower said plant he requires atleast 1000w , yet the plant will most likely yield less than 500grams. Those 500 grams could be easily achieved under one ebb table and a 600w SOG grow. The SOG also has the advantage of taking significantly less time and consumes significantly less electricity in the end.
Most growers find SOG/SCROG to be the most efficient method of growing both in terms of space and light and advanced growers are capable of producing truckloads of buds from relatively little light ,- thus after 3 months of flowering they will have more grams per wattage.
And example of this would be M.Blaze and his humongous trees,