Thinking the same thing. But now that I think about it, there is a lot of vegetation that would take up the majority of that share. Think about all the streama and rivers, then think about all the vegetation that surrounds and how easy it would be to fill those rivers with it. But still that's a lot of water.
Could just be that whatever was used to create the diagram has a minimum slice size and anything below that just gets rounded up. Without labels for the size of each slice it's impossible to tell.