During the print production and cutting of the sheets, these can move slightly. (This is normal) If you are seeing an un-even border it will be due to a combination of the image placement and movement of the sheet during the production.
Any images that are sitting 'between' the GREEN and RED dotted guidelines may fall foul of this particular issue.
1 - The RED dotted line is the final trim - With paper movement and images sat past/outside the red lines this would not occur as any paper movement and the position of the images would then be cut off - the result is images 'bleeding' of the edge of the page.
2 - The GREEN dotted line is the 'safe area' - With paper movement and images sat meeting the green dotted line, any movement is less noticeable as the safe margin is larger. Although under closer inspection these borders may not be 'exact in width' as there is more space either side the movement is less noticeable.
Wherever possible we recommend images to sit past the RED dotted line and meet the black outer guideline, this allows then 3mm of 'bleed' to compensate any movement during the print and production run.
With images sat against the green line and the movement of a sheet - the margin is less notable. This is why we provide this guideline as a safe area.