Cause the plant is growing still and producing flowers and bud and leaves etc, if you harvest to late your thc will be degraded as in amber tricomes, so its not always producing more thc under light. Its just that its full potential from the genetics can come out under great lighting conditions.
UV light, aka sunlight degrades THC. Keeping them in UV protected glass jars if lighting is an issue, but who keeps their stash out in the open in the sun? lol
A proper dry and Good cure is very important for quality of smoke, if you dry correct and cure properly, and leave out of UV light, the thc will stay fresh for a very long while.
After harvest or as soon as you chop anything down be it cannabis, strawberries, or lettuce, the item will degrade, thats why we put fruits in fridge's. all we can do as growers is maintain it the best from not degrading i.e. proper dry cure and storage
be well
FJG