AD sync clarification

Hi All,

           I am looking some clarification on AD sync:

Case 1:

1. I have synced Q1IM users with AD users.

2. All matching users (personnelnumber =EmployeeId) linked

3. From Manager, unlinked one AD user (opened Manager --> Active Directory --> User Accounts, selected one user and removed UID_Person)

4. Then started AD Sync Job (my sync configuration, selected Users but NO  'Change only')

5. After syncronization job, AD user was not linked to Q1IM user.

Case 2:

1. Executed above 1 - 3 steps

2. Went to native Active Directory and changed some attribute (i.e phone number)

3. Started AD sync job

4. After synchronization job, AD user linked with Q1IM user.

Why in Case 1 user is not linked? am I missed any configuration?

thanks

Rob