Pass sys_id or Name for reference fields?

[Suggy]

I have written Before Insert Business rule on Incident form.

(function executeRule(current, previous /*null when async*/) {

     current.assignment_group = 'Analytics';
     current.assigned_to = 'Luke Wilson';
})(current, previous);

But once the incident is created, this is how it looks:

As seen above, 'Assingned to' is set to the right value from business rule, but why not for 'Assignment group'?

- If i pass the sys_id for assignment group, then it works

- but for Assigned to field, if I pass sys_id/user name/user ID.... still it works.

What is the issue?

Note - Display value is set to FALSE for ALL the fields for both User and Group table.

[Suggy]

Anyone?