The aspect to look at is continuity and carry over. We talk about most rookies needing at least one (if not two) years to truly find their role, their fit, and to what level they can contribute. Coaching staffs are the same way. This also allows for certain variables to change allowing for a much wider depth of analysis.
Year one is establishment and familiarization...although there can be trends both positive and negative to examine. Year two, the majority of the team is now familiar so outsiders (FAs, rookies) have a wide range of people to emulate/follow and systemic integration is faster (again whether positive or negative).
I think this is why Kraft has shown a set three year evaluation process before firing because he realizes if it's blown up in year one there is no insta-fix - the process resets and starts all over again. The entire team goes back to zero...new buy-in, new familiarization, new roles and responsibilities, etc.