When I create a new cost plan (based on assignments - 'New from Task Assignments' action), the amount of the Planned cost is different depending on the Period Type used in creating it, whether it is Monthly or Annually.
How can you explain this?
Populate from 'Task Assignments' / 'Investment Team' work by populating the cost/rate values for each fiscal Time period and when the plan Period Type is Annually, the first day of the year as defined in the fiscal time period will be considered for picking the cost/rate for the entire year. If such rate is missing for that particular day then the default cost/rate would be picked.
For a Monthly plan, this would mean that for each month calculation, the start date of the month will be used for picking the cost/rate.
Therefore if you have monthly cost/rate changing over the course of the year then you are likely to see the discrepancy in the Planned Cost values depending on the Period Type used.