When creating mathematical models for a simulation, it's almost impossible to account for every possible variable and some assumptions have to be made. Once a model is created, it has to be validated against real data to judge its accuracy. Since this is Tesla's first car they had to create all their simulation models from scratch and had no data to validate against until prototypes were ready to be tested. Part of Simpson's job is to use actual vehicle data to improve those models and insure that future estimates are more accurate. To find out more about some of the design decisions the Tesla team made, check out Andrew's post.
[Source: Tesla Motors]