
From my other post (Cold-pressed/pellet food manufacturers in UK) there is an explanation of the difference between cold-pressed and extruded foods.
From what I can make out the use of a low temperature (below 75ºC) during production prevents the ingredients from damage or nutrient loss due to excess heating (extruded food is cooked at over 100ºC). Also the way it digests is different. Have a look at
http://www.farmfood.co.uk/dogfood/premium-quality/natural-holistic/why-pressed-pellets.html which shows a comparison between extruded and cold-pressed foods when soaked in warm water/vinegar.