I think it because they have asked you for that police certificate. In the email you got is there a time period mentioned, like 30 days or something submit the certificate? Most probably you will have a new BF Date based on that. In your ECAS, when you click In Process does it say which date they started processing your file?
when they sent me the email, i got 3 mos (90 days) to comply sending to them the documents.
when it changed to IN PROCESS, it says We started to process your application file on May 21, 2012.