I think I have found the answer: MATLAB appears to be reporting the hexadecimal values '3004' and 'C' as decimal. ( Bing provides a hexadecimal conversion tool.)
Compare Page 611 of the PS3.3 2016b "part03.pdf", i.e. the section C.8.8.3.2 Grid Frame Offset Vector.
So in this case the Frame Increment Pointer is identifying the Grid Frame Offset Vector, but is reporting the tags in decimal rather than the hexadecimal used by the DICOM Standard. That is my surmise.