ps -o affects reported values?

[Jo Schulze]

While working on a munin plugin, I noticed some IMHO strange side-effect
of the -o option:

ps -e --sort=uid -o euid,comm,user

works as expected:

[...]
  100 dbus-daemon     messagebus
  101 hald            haldaemon
  101 hald-addon-acpi haldaemon
[...]

ps -e --sort=uid -o euid,user,comm

on the same machine displays the uid instead of the user column for 2
users:

[...]
  100 100      dbus-daemon
  101 101      hald
  101 101      hald-addon-acpi
[...]

1) Can anyone reproduce this behaviour?
2) Is there any reason for this or is it just a bug?

System is
procps-3.2.7 
Linux, openSUSE 10.3 x86_64

TIA