I guess is because you have to actively listen to the content on your mobile phone, ipod, or whatever other source. You are more concentrated because audio is your only source. While being on the PC / iPad or watching TV it can easily happen to become distracted, so you pay less attention to the dialogs, thus loosing content that you might otherwise understand. If this is not the case, try to change the audio mode from surround to stereo or even mono, to see if you understand more or less.

The real experience will be different anyway. Watch some of the youtube videos, with crowds of people, all the noises and sounds, this can't be simulated or improved or filtered out. So in the real world you might or might not have a problem in understanding what other people say. This is why I think you shouldn't worry about such minor problems too much.

I don't think it's that much of a mystery.  Almost any place will have a good degree of backround noise.  Right now I have a chorus of frogs and a humming refrigerator reducing the audibility of my TV, and if my wife is running water in the sink, fuhgeddabout it.

With headphones, the audio is pumped directly into your ears.  For me, if I'm listening to the TV with headphones, even stuff i would normally just ignore is almost too clear not to understand.

Anyway, back to the original topic: I suggest that you not sweat it too much.  If you're talking directly to someone, often they won't talk at you with the jackhammer speeds that people on TV do, and if you miss something, you can always ask them to repeat themselves.  Shouldn't be that bad.