Challenge 37 problem - Puzzle 37 - Turing Tumble Community

Hi Guys, First of all, I am not a beta kind of person so I apologize for my question ;-). This is my outcome on 7+9 in challenge 37. I thought register B would indicate 15 and then put one marble in the overflow bin. But apparently I’m doing something wrong or making a logical fallacy. Can someone help me out? Thanks in advance!

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