Each time you add a plugin, another slot will appear. One slot doesn't mean you can only put one plugin on that channel. There's really no need to have a bunch of empty slots, but i guess if that helps your creative process, who am I to say it's a bad idea...

The only method I can think of would be: (For Example) If you had an EQ in the first slot and a Compressor in the 2nd, you could take the Compressor and drag it down a few slots to create empty space for more plugins. Here are pictures to show what I mean:

Before: http://imgur.com/bf1HALW

After: http://imgur.com/2vU4yZR


I think the second photo you included may have been a bug, simply because the empty slots *below* the final insert in a signal chain will typically disappear once you get rid of that insert.

Hope this answers your question!