This. You always find out new stuff during the development. Nothing ever goes according to the plan.

I have a favourite illustration of the way projects work, right at the top of this page: https://bureau.ru/about/fff/

(The page itself is in Russian. Basically, it’s about the “flexible scope” principle from, you guessed it, Getting Real: https://basecamp.com/gettingreal/02.4-fix-time-and-budget-fl...)