If you are using full bleed images, Atticus will automatically add 3mm to the top, outer, and bottom edges of the page.
Unfortunately, I'm not entirely sure how BookBaby would handle our formatting at this time because it's not industry standard to add a trim allowance to the inside edge, but this is why their width requirement is 3mm/ about 0.125 inches larger than what Atticus produces or what other distributors and printers require. Atticus is not currently programmed to handle custom bleed allowances, so this is something that would have to be added with a PDF editor like Adobe.
I recommend reaching out to BookBaby directly for further clarification on their specific requirements and how they handle the trim allowance. They will be able to provide you with the most accurate information regarding their printing process and any adjustments they make during production.