"The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time."
80% of the work is completed in 20% of the total development time; while the remaining 20% of work consumes 80% of the time.
And this is something I've seen again and again: the large, most impactful features and architecture of a system are designed and implemented fairly quickly; it's the many small details get forever to be defined and implemented accurately.
Now if 90/90 is accurate because of unknown unknowns, or just because we are all terrible at estimation and planning is debatable;)