Featured image used as header image sometimes?

Hi,
On my site, http://www.autocord.com, I simple posts with one image, and I mark them as featured image in case I will use another theme later where that might be useful.

Well, my question is: Why does my most recent post (http://www.autocord.com/yosemite-mirror-lake-wit-autocord-3-d-lens/) use the featured image as heading while none of my other posts do?

The only thing I may have done differently is that with my most recent post I marked the image as featured image from the start but not afterwards. Still, I’m not able to find any setting to modify this.

Agust