Estimating development time is similar to the coastline paradox. The more detailed your project description gets, the longer you estimation will be. You start with “building a blog will take me 20 hours”, then go to “OK actually I will need 10 hours for the data structure, 10 hours for a way to create posts and 10 hours for the actual website”. For each of this estimations you will end up with more hours needed the further you break them down.