I was wondering just the same thing. I can build features and ship code, but I'm definitely slower than my co-worker (who has been on the project for over 1 year, while I'm just in my first 3 months).
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.
How can we measure where we fit on the scale?