Google prefers to not share long tail searches in this tool – because the tool is designed for Google Adwords advertisers. If advertisers bid on those long tails – they would have less competition and therefore pay less for the Adword, and therefore Google would make less money e.g. bidding on the term weight loss for girls under 18 is cheaper than weight loss.
How do I know that the KC score measures competition accurately? When I first started using the KC feature – I didn’t know for sure. But I’ve been using it since December of 2012. I’ve published literally hundreds of different articles (in different niches) around keywords with low to medium KC scores and I have a pretty good feel for the accuracy. More than 5 years of successfully ranking for low competition keywords has convinced me to completely trust the KC scores that are calculated within Long Tail Pro.