This problem of labelling comes about because the word 'modern' in art was used for some art from the mid-Victorian period through the whole 20th century. It covers a wide range of styles and movements. So when art critics wanted to describe post-WWII art they had to use a different word. Thus, in art, 'contemporary' (which really means 'at the same time as') comes to mean 'modern', and modern comes to mean 'earlier'.