That looks to be because the jack is an AV jack, not just a simple headphone jack, where the connectors are arranged such that when you plug headphones into it they only make connection with the audio part but when you have the right cable (with a much longer connector with more segments) it would make connection with both audio and video out, but each have their own separate physical connectors.
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I know on the old iPod's with video out you could get an RCA cable that went from 3.5 mm to AV cables:
http://www.macworld.com/article/149335/2005/11/apple_ipod_av...
That looks to be because the jack is an AV jack, not just a simple headphone jack, where the connectors are arranged such that when you plug headphones into it they only make connection with the audio part but when you have the right cable (with a much longer connector with more segments) it would make connection with both audio and video out, but each have their own separate physical connectors.