Just to put my 2 cents into the overall conversation on this thread
Do some of the issues we see with the power looking to change from the early books to later books revolve around Rand and Ishamael?
If so, is there potential that rather than the power changing, that Rand and Ishamael being the champions of light and shadow respectively, have a general innate strength, equivalent to others of 1++ strength, but that at times they become conduits of light / shadow, and so their apparent strength is even higher at those times?
I see this potentially supported by Rand's view of the forsaken in the first book, with the threads they had to the shadow and the differences between Aginor's and Ishamael's, and Rand's display against the shadow at Tarwin's Gap in the first book as well.
And let's not forget that Ishamael wasn't actually using the OP all of the time. He utilized the True Power most of the time and that seems to involve being as strong as the Dark One allows you to be rather than some innate strength level.
Moghedian's boost in strength at the end of the last battle is strong evidence of this I think.
Quiet possibly ... Either that or it's another one of those Sandersonian quirks
Unfortunately one of those things we probably won't know for certain, as certainly a few things that I think he got wrong in the last books.