There is a minimum light threshold. The best way I can explain it is this:
If I had a coffee mug with java moss in it and could put a 1 watt fluorescent bulb over it, if you figure the coffee mug holds 16 ounces, ( big cup), then you have 1 watt over 1/8 of a gallon or 8 watts per gallon. Wow, that java moss should really be growing with all of that light now, eh? It won't. More likely it'll die and turn into mush. That 1 watt bulb cannot provide enough light intensity for that moss no matter what you do to that bulb, even though you have " 8 watts per gallon". If a plant has to have "xyz" amount of intensity and the bulb can only provide "x" , then it doesn't matter how many watts per gallon it is.
That is why on smaller tanks, the WPG rule is out the window. On very large tanks, it is also out the window. You'll get the most out of the WPG rule with tanks 20-90 gallons (approximately).