Could be that Dylan was prepared to be adventurous with his guitar style in the studio where he had the luxury of practice time and as many takes as he needed to get things right. In Newport in front of an audience and being filmed for posterity he may have resorted to a simpler guitar style that was less risky in terms of getting things completely wrong.

Also given that the sound equipment, mikes, amplifiers etc were a lot more primitive in those days he may have wanted to make an impact with his guitar playing. Subtelty could well be lost if the reporduction was not good. Although the sound on the recording of Newport seemed OK, the sound in the 'house' may not have been the best - particularly as the 'house' seems to have been outdoors a lot of the time.