A bunch of users are confused by the fact that limits they set for attachments aren't working. This is because they are not aware of the fact that administrators and forum moderators are not affected by some limits, e.g. maximum file size, maximum image dimensions, maximum number of attachments per post/PM. There were several tickets filed about that (f.e. http://tracker.phpbb.com/browse/PHPBB3-8508, http://tracker.phpbb.com/browse/PHPBB3-7204, http://tracker.phpbb.com/browse/PHPBB3-7515 and more), which were closed as invalid.
I believe it either should be documented on the corresponding ACP page explanation (`Attachment settings` ACP page) and/or special switch/permission should be added to allow/disallow admins and moderators to ignore those limits.