Praneeth,
In reports and in Forecast in the new UX this is calculated as actuals to date + remaining forecast of the project.
If you are trying to look at spend to plan you lose that when creating new plans each month.
Based on the Financial Forecast Review by Investment
Forecast (Calculated) This is the actual cost posted through the prior fiscal period plus the planned cost beginning in the current fiscal period.
Forecast Remaining This is the actual cost posted through the last complete fiscal period plus the planned cost remaining in the cost plan of record after the last complete fiscal period. For example, if the project's as of date is 05/20/2015, then forecast remaining is actual cost through 04/30/2015 plus planned cost in fiscal periods after the fiscal period ending 04/30/2015.
Financial Forecast Review by Investment - CA PPM - 15.6 - CA Technologies Documentation
In the new UX the definition is that same:
Under Analyze Cost Plans at