Poster Paint

I find it very interesting how they call poster paint or children's paint by the name of tempera paints as if they are the same thing. In reality they are very different. For instance tempera paint uses egg yolks as a binder when according to this website, http://www.ehow.com/how_2097379_make-poster-paint.html, you use flour, liquid starch, and liquid detergent for the binder effect. As well you can only use tempera one time, you can't just put it in a jar and use it another time. With children's paints you can reuse them and store them in containers and they will be just fine. I still dont see how the two get classed together when they are so different.