U-Source ideographs mapped to themselves

[Uriah Eisenstein]

Hi,
UAX #38 (Unihan) defines the kIRG_USource field as a reference into the
U-source ideograph database described in UTR #45, having the form "UTC
nnnnn". However, several CJK Compatibility Ideographs are mapped to their
own code point values, e.g. "U+FA0C    kIRG_USource    U+FA0C". The formal
syntax of kIRG_USource allows this, but I've found no explanation as to the
meaning of such a mapping; there is also no such mapping from a code point
to another code point.
Thanks,
Uriah