Updated an automated workflow for new leads, now it's running on existing leads

I just updated an automated workflow that assigns New Leads to my salespeople.  The update was just administrative (email content and products etc).

After updating and activating it, I went on to do some work cleaning up our accounts (also administrative, eg merging or updating information).  

Now my salespeople are receiving lead assignments for existing records, and I realise that the workflow is kicking in whenever I update an account that has an open Opportunity tagged to it.

this workflow is for Lead entity only, and is supposed to start when a record is created / assigned (ie Totally new leads only).  Can anyone tell me where I'm going wrong on this?