Workflow assigning Opportunity record to wrong User

Hi, 

I have a workflow that runs off of a custom field 'Walk-In' which is in the process bar.  When this field (date and time) is selected, and the Contact contains data, the workflow does 3 things:
1.  Updates the Contact type from (custom field, checkbox) Lead to (custom field, checkbox) Prospect

2.  Updates a (custom field, date) Date of Last Visit to the Modified On Date

both of these are working as they should

3.  Assign to User who selected the 'Walk-In' (Assign:  Opportunity to Opportunity:Modified By)

This keeps assigning the Opportunity to the SysAdmin User (Z2T)

The workflow appears to be correct - could this be a security permissions issue?

Any help is appreciated.  Screenshots attached.