Software development is not that easy. Sometimes you have a problem that you think can be solved in 5 minutes and then you spend a week working on it. Or one problem leads to another one, and so on, etc.. that's why hardly any developer agrees to fixed deadlines or if so, then it often comes down to crunch times.
It's done when it's done.

or another saying (bit sarcastic) : The last 5% of a software project often takes as long as the first 95%

edit2: or in the case of games in general: you have a plan, you make the game and in the middle or end of the process you see the features are no fun. so you adjust and adjust and adjust till it is fun. fun cannot be planned, fun has to be experienced.