Query on Business rule wrt Assignemnt group and Assigned to fields?

[Suggy]

In user table I have an user called'UserA'

In group table I have a group called 'GroupA'

In my business rule, i have written below 2 lines of code..

current.assigned_to =  "UserA"; -----> THIS WORKS

current.assignment_group = "GroupA"; ------> This DOES not work... WHY? (if I pass the sys_id, only then it works).

Why? both are reference fields. Then why the difference?

[Hugo Barros]

@Suggy, i was thinking about this and i have a question: When populating assigned_to, have you checked if the field references the right record?