If you have images set to bleed in your PDF, Atticus automatically adds an additional .125 inches (3.175 mm) to the top, bottom, and outer edge of your pages. This extra allowance for bleed ensures that the content extends beyond the page edge to accommodate trimming during the printing process.
Since this area will be trimmed off, this bleed area is not considered within your specified margin settings, which might result in the appearance of uneven margins.