In concordance with that post, think of programming as writing a large novel. Sometimes a paragraph you wrote just doesn't work, or contradicts something you wrote before, and has to be rewritten. Sometimes an entire chapter leads somewhere you didn't intend. Other times, you take a certain approach and reach a desired conclusion, but it makes another part of the story unworkable, and you have to rewrite several chapters from several subplots of your story. And other times, the words seem like they are begging to crawl off your fingers an onto the page.

The same is true of a computer program; just like a novel, it's got several subplots - a system for villager health, recording it, ways of affecting it, ways of being hurt and healed. That plot has to agree with the plot for skills, where a villager may have an accident in the use of a skill and hurt themselves, or through the use of healing, be cured of sickness. (and stop losing health.) If any one of these subplots refuses to agree, steps may have to be retraced unpredictably so that a complete, coherent, functional story - or computer program - can be written.

Programmer's and author's estimates are wrong most of the time because these things can't be predicted, to the frustration of their publishers.