The main reason for changing the task dates is that performing risk analysis on a project schedule requires that the schedule tasks need to start as soon as possible, while Primavera P6 and MS Project allow tasks to 'hang' with manually defined start and finish dates. Tamara recalculates such tasks and assigns their start dates to earliest possible (i.e. converts them to 'auto-schedule' mode). Hence the differences in the original and imported schedules.