Hi daviddekleer.
The problem is that SOngs can go away. In the case you point out, SOVHRUI12AB018309A was merged into SOOHRBD1311AFE26A0 back in 2012, presumably because we got a new track whose audio was close enough to both other fingerprint clusters to cause us to merge them into one big song.
The API will continue to work with merged-away IDs, but we wouldn't ever return them from things like song/search.
(For what it's worth, the Million Song Dataset suffers from the same problem: many of the SOIDs in there may not be the same ones you'd get back from the API today.)
-Eli