Probably some problems with overlapping triangles. Check your indata, crossing breaklines may cause the problem. The original surface should be larger than the design so that is correct.
The "Almost Exact" option projects the design triangles to the original surface and then add all projected triangles as breaklines to the original surface and creates new triangles. This method makes it possible to create prismoids that match both surfaces. The error should contain a value of the size of the mismatching area, if this is small you may consider if you can use the volume anyway.