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When I have 2 email lists with a difference of 1 contact (or any other number) it would be very useful to see what the difference is in contact records. (Especially since I thought I just added ALL active contacts to the list but I'm off by one.)
Thanks for sharing this feedback! Can you try copying all of the contacts from list A to list B to confirm they have all been added? Also, I am wondering if there is a contact with more than one email address that could account for the difference.