The limit isn't a hard limit, but is a soft limit. The way the feature works is if any image group has LESS than that number of items with that image types priority level or less than the download is allowed.
For example:
Your download limit is: 1
Your Box Front Image Priorities is set up as: Box - Front, Box Front - Reconstructed, Fanart - Box Front
Your game already has the following images: 1 Box Front - Reconstructed
The process would download a Box - Front image once (since it is higher priority than the reconstructed you already have) and skip any other type in that group since you already are at the limit.
If an image belongs to a type that DOESN'T belong to an image group, the limit is applied to ONLY that image type. So how the limiter behaves greatly depends on how you have your image group priorities set.