Hi Joe,
Yes, I think you've got all that right. The mosaic center is derived from data that is always in the DICOM header, but Siemens doesn't always put slice information. I can not say why that is. If slice information is available, we use that to confirm the mosaic center. In our initial tests with oblique data, they were always the same if the slice data did exist. As you have seen, it may be different. In that case, the warning appears.
Because there is no available specification for Siemens implementation of mosaic format, we can not really pretend to completely understand the logic of this format. It's best to be aware that there could be a misinterpretation of the Siemens DICOM header information. A simple way to check for this is, as you suggested, to make sure the data aligns well with another reference dataset.
