"Prevent computer from Sleeping automatically when display is off" activating by itself

I have a iMac 27" retina using current version of el Capitan. I've noticed that the "Prevent computer from Sleeping automatically when display is off" self ticks stopping my iMac from sleeping.

By self tick I mean that I can leave the computer with the box in energy saver unticked, put it to sleep manually but when I return the computer is running with the display off. When checking the energy saver the "Prevent computer from Sleeping automatically when display is off" button is ticked?

Nobody else uses the computer but me. Something is causing the button to tick and stops the iMac sleeping properly as it should.

I've only noticed this with El Capitan, it did not do this with Yosemite.

Anybody else experience this or is there a fix?

Hardware is the i5, R9 M290X GPU, 16 GB RAM, 1TB Fusion. Late 2014.