Why is the order of the legend items on the exported legend pages different to the order of the layers exported in the map?

Layers might contain many features with different symbology. During the export of legend pages the features of layers are kept together and are not distributed over multiple legend pages. If the legend items of a layer's features do not fit in the defined legend element, legend items might be truncated. The order of the exported layer's legend items might also differ to the order of the layers in the map, since layers with less legend items are exported in the way they fit best on the available legend pages. In this way the number of legend pages will be as small as possible.