Just to clarify here so there is no confusion to the community. As these particular images are Gallery images, the original resolution file is kept. This is due to in Gallery, the quality/resolution of an image is of utmost importance so if we do anything to the image in settings that requires a rebuild of any of the thumbnails we display, we require that original image to do that (otherwise, technically speaking there are a lot of compromises or incompatibilities that would need to be accounted for).
Normal attachments anywhere else in our software do not follow this technical design and follow the settings mentioned at the top of this topic in ACP -> System -> Posting.
Of course, if you disagree with this technical design, please leave your feedback in our official Feature Suggestion forum for further evaluation.