EMGs/NC's only look for damaged muscle or nerves. If not enough damage has occurred, it comes back "normal." There is a wide range of normal so even slight variances are considered "normal.
My first NC showed I had slowing of the ulnar nerve. I was dx with TOS at that point (2008) but was told that it was not the cause of the damage and it was probably from an ulna/radius break I had when I was 4. I had another NC in 2012 and was told I did not have TOS and it was completely normal and they didn't see any slowing on the ulnar nerve at all. Then, I was rediagnosed with TOS after a doppler ultrasound and was told the NC didn't matter at all! lol.