Why does my Budget at Completion (BAC) differ from my Estimate to Completion (ETC)?

As for the budget at completion being different than the estimate at completion, they are based off different variables. Budget at Completion (BAC) is pulling from baseline data. It is adding up all the baseline costs (labor, non-labor, material, & expense). Estimate to Complete (ETC) is pulling in actual costs.  The two could always be different.