In my experience, pipeline/forecast is one area of a CRM system that changes frequently. Way to frequently.
Fortunately, these changes require less coding changes today through Windows Workflow Foundation (Dynamics CRM) and Workflow Automation (Salesforce) that in CRM days of old.
Not going to comment on whether it is right or wrong, just that it happens, often, as a matter of fact in many organizations. Change Sales VP, there will be a change to the sales process. Change the compensation model and the forecasts will change. It happens all the time and makes numbers from sales very short term and difficult to do long term trends. The best way to improve accuracy, find a sales process that works and then measure the core of that process. Less change, more value.