The process of artistic growth -- particular an increase in sheer technical skill -- is at odds with the desire for consistency in style and surface across an artistic work.
For shorter works, obviously, this is less of an issue -- one of the many reasons beginning writers are encouraged to write short stories. You learn a bit, finish a piece, and apply what you've learned to the next. In the idealization (almost certainly rarely if ever achieved in real life, like the straight line through the vaguely-linear scattershot of data points), each story is better than the next.
But for a longer work -- the kind that takes years to create -- it's more difficult. Here, if you get better -- and again, I'm thinking primarily of sheer technical skill here -- you get the odd situation where the ending is better done than the beginning. Oh, you can go and rewrite the beginning... but it's time-consuming, and it's a process that lends itself to indefinite extension (although the point of diminishing returns surely kicks in somewhere). Sure, if you've done a lot of shorter works first this is probably less of an issue (although I suspect that artists rarely work so long on a short scale that it's not an issue at all when they turn at last to the long). But what if you are drawn to longer works, for whatever reason? Then it's a bit of a dilemma -- or at least a problem.