I was so sorry to hear your news. I can only guess at the panel's reasons.
For what it's worth, I'll give my own view. You had a good solid case but (as I think you yourself said) nothing that made it stand out. If only the head could have been persuaded to predict 5a's .......... (before everyone else panics, I'm not saying this is essential, but in such a borderline case it could have tipped the balance).
The 11+ is based on VR and, personally, I think the two previous VR scores would have been a definite minus. They may have been a year or two old, but they're on the record, and then the 11+ provided two more opportunities .......
There were inaccuracies in the head's rankings. Some unranked children qualified. Moreover it didn't help that the two immediately below you "missed by miles". If they had actually qualified, it would have made your son stand out as an anomaly.
I do hope the panel were not influenced by lack of tidiness in the exercise books. This has nothing to do with academic ability.
I hope too that the lack of extenuating circumstances were not an issue. With a score of 120 that would be very harsh.
I said beforehand that I didn't think you would have a lot of questions, because the facts of your case seemed very straightforward. It doesn't surprise me, therefore, that the questions appeared lightweight, and that there were a lot of "fillers".
I don't believe for one moment that the panel had made its mind up in advance - that simply isn't the sort of thing local authority panels do.
Having said that, panel members are fallible, especially with borderline cases where the decision could go either way. It's like the courts - a different judge/jury on a different day might come up with a different result. I know it's no consolation.
You've been so supportive of others on this forum, Ambridge. Please don't lose heart. I believe your son should be at a grammar school, and I hope you will give serious consideration to the 12+.