Basically, several things come in to play.

First, UK doesn't use models to show off their kilts. All pics are of actual customers (though there are a few of employees mixed in including many of Steve the founder), in every day situations, so everything is not always ironed and folded perfectally.

Second, as Rigged pointed out, UK's are basically jeans/kakhis, not dress items. While I have known guys that iron jeans, most do not. They are meant to be worked in, played in, hung out in. As a result they can be a little rumpled at times. The first UK was created while Steve was working on his motercycle, I don't think he was worried about being unironed at the time.

Finally on length. Speaking as someone who has sold UK's at festivals, mid kneecap is the target length, same length as I was measured for for my tank in Glasgow at Thomas Gordon and Sons. Now UK only comes in standard lengths so hitting the exact spot is not always possible. I have found that if given the choice most guys go for a little longer than a little shorter (espicially younger guys or first time wearers that are a little nervous in the first place). I don't know the reason, but I suspect it is because shorts that are popular in the US right now also tend to hit just below the knee. I've actually had guys say, its not a traditional, so I can wear it a little longer.

Some of it is just perception of the camera though. Again most pics on the UK site are snap shots. Look at your own pics where the person holding the camera is standing and not really worried about shooting the kilt to make it look its best - instead trying to get a pic of you. The kilt looks a little long. Now look at a pic where the person is kneeling and trying to get a pic of the kilt in all it's glory. The kilt looks the right length.

Adam