Baseline Duration custom field not calculating duration properly

Hi,

I am using MS Project server 2010. In it, I created one custom field named 'Baseline_Duration' similar to 'Baseline Duration' & applied the formula 'ProjDateDiff([Baseline Start], [Baseline Finish], "Standard")'. In Baseline_Duration& Baseline Duration columns, values should be same. But for some tasks, values in Baseline_Duration column is different from Baseline Duration column, & for other tasks, both are same.

Please suggest me why values in both columns are different for some tasks?

Thanks & Regards Pradeep Gangwar