It is the usual complexity rule of software: solving 80% of the problem is usually pretty easy at only takes about 50% of the estimated effort, it is the remaining 20% that takes up the remaining 90% of estimated effort (thus the usual schedule overruns).

The interesting thing is that there are problems for which this rule applies recursively. Of the remaining 20%, most of it is easier than the remaining 20% of what is left.

Most software ships without dealing with that remaining 20%, and largely that is OK; it is not OK for safety critical systems though.