When you click on the scene selector on the bottom, then you will see it's properties on the right. In there there is a beginning and end distance, those are controlling when that scene can be displayed. Value of -1 means infinite, so those scenes will be endlessly displayed until game over... Scenes that are in the same range are randomly selected so that gives the variation during game play.
Typically there is a Start scene which starts from 0 and the end set to 1, and all others start from 1 and above and the end is -1 or some bigger number. This way Start scene always runs at the beginning and no other scenes start instead and Start scene runs only once simply because the end distance is 1 pixels so no way that it will be picked once more... But if you start a new project you should have this setup already for you, two scenes one Start and one called "1" by default - this latter one is already set for endless. If you click that and press D on your keyboard it duplicates it and therefore you will get a scene 2 with same settings and whatever objects were already there are also duplicated over to the new scene. So you can create and modify existing scenes very quickly.
as a side note to @trudnai 's help.. when you set your game to be endless like that, the play cooldown setting in the options panel when you select a scene is how many player deaths before that scene repeats. edit: heh...1500th
