Nothing.

Quality Control in the manufacture of diskettes was all over the place.

Reversible was an assurance that both sides were the same quality. If it didn't say reversible, only the top side was claimed to meet the manufacturer's standards for reliability. The bottom side might well still be usable, but more likely to have defects of consequence. A reversible disk typically had a glossy, protective coating on both sides instead of just the top.

The coercivity and structure of the actual magnetic coatings, though, were identical for SD and DD disks, so you could use either type of disk in an SD or DD drive. However, DD drives formatted data in a different way to achieve higher capacity, so were more sensitive to disks' manufacturing defects.

SD-labeled disks were more likely to have defects which caused problems in DD drives. DD-labeled disks were relatively defect-free and thus much more reliable in DD drives.

The Commodore 64's 1541 was an SD drive that was notoriously sensitive; it worked much better with DD disks.

Some manufacturers strived to make all of their disks DD-quality, and would just label the slightly defective batches SD. So an SD box made in the DD era could well be full of DD-quality disks, or SD-quality, or a mix.