One reason for that is of course that it is not like similar applications, but of course also because that the authors were not sufficiently aware of the importance of using a consistent vocabulary. Usability literature tells us that it is very confusing to the user if the same concept is described using different words in different contexts.
It is an ongoing process to unify KPhotoAlbum to a common vocabulary, so if there is a concept not described here, then please discuss it on the mailing list and/or IRC channel, and then later add it here.
Consider the suggestion from bug #145530
1.5.8