The "Gap" at the backing ring may very well hold some slag however that slag is not what shows up in a radiograph.
It is the internal undercut that is not filled in with weld. Regardless of the fit of the backing, allowing the root to properly fill in will help eliminate this issue.
Slag inclusions ,which are normally dark on film. are not dark because it is there. Its dark because there is no steel there. If the gap between backing and pipe were smaller you would have less chance of undercutting the internal surface.
Both of the illustrations below would look nearly identical on film. A radiograph cannot tell you if there is slag filling a void.
Be sure to pause on the sides long enough to let the weld fill in. Its easy to think because there is backing, hotter is better. That may not be the case.
Even with fill passes the leading cause of trapped slag is NOT the slag that was left but the contour of the weld prior to depositing an additional pass. A radiographer cannot tell the difference between a void in the metal that has trapped slag or one that does not.
Have a nice day
Gerald Austin
http://weldingdata.com/slag_nonfusion.htm