UID was never a lable for uniqueIdentifier. However, LDAP and certain

RFCs concerning X.500 directories use UID as a shorter name for the
attribute type userId, which is defined by CCITT and available through
RFCs 1274 and 2247.

Unfortunately, if some applications have used the name "UID" for the
uniqueIdentifier attribute type, they will produce incorrect results.
However, I found it better to follow the standards that are out there
rather than having our own incompatible one.