It's deliberately like this as true random ordering in MySQL gives awful performance.
In more detail; we actually pick a random offset and then show X media items from that offset. So, it is random, but you will indeed likely see a selection of images from the same album.
A way around this (sort of) is to choose a larger number of images to show on the slider. So instead of 5 or 10, consider 20 or 30. The images picked are then shuffled, so if you happen to select media items that might span multiple albums, it may appear to be slightly more random.