I saw virtually none of today in court. Roux's whining caused me to switch off both TV and Audio. I did pop in here to catch up but I haven't seen anything about the number of times Nel was accused by Roux of calling OP a liar. Roux intimated he was going to bring up the issue today and quoted many pages where he said Nel called OP a liar. Did Roux bring the issue up, anyone? As far as I am aware Nel never said liar, only lying on one occasion. I am sure I heard Roux say exactly that to the earwitnesses when he was cross examining.
I'm not sure if this is the legal reason, but if I call you a liar, I attack your character. If I say that you are lying, I attack the factual content of your statement.
AFAIK Nel said Oscar was lying many times during the case. These were allowed by the judge. But the one time Nel called Oscar a liar the judge called Nel on it and he immediately apologised.
Edit: See post below. Interested Bystander correctly said that Nel never used the words "You are a liar." The link to what happened is in my answer a few posts down.