Values from IfmGetTotalContent

Hello,
I have one question.
When I calculate the volume of the field manually, gets the same value as when I call the function IfmGetElementalContent (with parameter IfmContentType=IfmTOTAL_VOLUME) and summed these values in all elements.
But when calling the function IfmGetTotalContent (with parameter IfmContentType=IfmTOTAL_VOLUME) the resulting value is different from previous values (value is higher). Tested on more tasks.
Thank you.

->total volume elements (the field has 11 elements):
for (a = 0; a <= 10; a++) {
volume_t= volume_t + IfmGetElementalContent(pDoc,IfmContentType (type)=IfmTOTAL_VOLUME,nElement=a);
};

->total volume of the field:
volume_t2=IfmGetTotalContent(pDoc,IfmContentType (type)=IfmTOTAL_VOLUME )

volume_t  not equal to  volume_t2