Curiosity about DBA_AUDIT_TRAIL.OS_USERNAME

Oracle 12.1.0.2 SE-2

Another side curiosity while analyzing something ...

When querying DBA_AUDIT_TRAIL, I see the same OS username reported in different case formats:

SQL> break on upper_os_username
SQL> select upper(os_username) upper_os_username,
  2       os_username
  3  from dba_audit_trail
  4  where action=100
  5  and timestamp > sysdate - 180
  6  and upper(os_username) <> os_username
  7  group by upper(os_username),
  8       os_username
  9  order by upper(os_username),
10       os_username
11  ;

UPPER_OS_USERNAME         OS_USERNAME
------------------------- ---------------
ESTEVENS                  EStevens

                          estevens
JDOE                      JDoe
                          jdoe
SQLADMIN                  SQLAdmin
                          sqladmin

Notice

1) they all have a variant of all lower-case.

2) One has a variant of the first letter being upper-case