The first painting I came across in the Photography classification was neither keyworded or said to be a photograph. In fact, the artist puts its medium as Oil Painting. So the OP is correct, there are random other-media images contained within the wrong media grouping.
While many people would use the search feature, others might just want to look at one medium and up pops another. The other "problem" (it's not life or death) is that things that really do belong in a group are, conceivably, further back in the page sequence or might not be there at all. Who's going to click on "oil paintings" to try and find a photograph of a sunset? Conversely, if that painting is not in its correct group, it is not being seen by people only looking through that group.