Seeing cross browsser / device issues coupled with using User ID (no replies)

Hi all,
I am seeing some issues with using the UID in conjunction with cross browser/device scenarios.
Let me lay out the different scenarios and what I am seeing.

New user comes to the site ( no cookies ):
-clicks around (logging some page views - anonymously)
-registers (filling out a form)
-we pass in the userID
-all anonymous visits are then brought into the correct Users profile associated to his UID.

**this works perfectly. No issues here

---now---
Lets say that same user. closes chrome. opens firefox. (so no cookies are present)
-clicks around (logging some page views - anonymous - as he has no cookies)
-registers (filling out a form - with the same data as before - so "we" know who he is and attach that same UID to him)
-all the anonymous visits are kept in some separate profile view - while all the future clicks on the site are associated correctly with his UID profile view.

Not exactly sure what is going on here - but wondering if anyone else has run into this issue. known issue? any insight to this problem.

Please let me know if I confused anyone and need to re lay that out.

Thanks all
Citti.