A lot of the productivity difference is probably due to overall familiarity with the code base. In a year from now when you have to fix a bug or add a feature you will probably know where to go and have an idea of what needs to be done rather than having to go hunting for it or asking another developer. If a project is started from scratch I imagine a lot of difference in productivity is due to familiarity with the tools, frameworks, api's, etc. used.