The function exportgraphics generates and saves the image at the specified resolution, but it does not write resolution info into a JPEG file's metadata. The "96" you are seeing is probably just some default value that is populated in the image properties tab when a file does not contain that metadata.
Under the hood, exportgraphics uses imwrite to create TIFF, JPEG, PNG, and GIF files, and imwrite does not support resolution metadata for JPEG files. It does support resolution metadata for TIFF and PNG files, and so exportgraphics will write the specified resolution to a TIFF or PNG file.
The JPEG File Interchange Format (JFIF) spec does include a way to include resolution information, but I have the impression that resolution metadata is not as widely used for JPEG as it is for TIFF and PNG, which are more commonly used in print publishing applications than JPEG. JPEG is not as suitable for print publishing. If you are generating a 600 dpi file, then I imagine it is for a print publishing application, and then I would recommend using TIFF or PNG anyway, since JPEG compression is lossy.
I did find an enhancement request in our system for writing resolution info to JPEG files using imwrite. I'll add a note to it about this thread.
