What's frightening is the variability of daily productivity. Some days everything comes out the first time, you're using the right framework and you finish 20% of the project with 10 lines of code. Other days, you lose hours chasing down a bug from a single typo, or debugging an obscure corner case incompatibility between two libraries. And there's no sure way of telling what kind of day you'll get next.
Then the first derivative comes into play: some days you manage to keep it simple, other days you overcomplicate things. And there's no sure way of telling what kind of day you'll get next.
This tends to affect future work rather than the current sprint.
