I suggest you browse how Semrush, Ubersuggest calculate their KD.
For Ahref, it's about the number of backlinks to each individual page on Google's first page.
If I can recall, Semrush is about the number of sites RANKING or Competing for that particular KW.
I don't know about Neil Patel Uber..
But I for sure will go with Semrush over any other, Ahref is cool too, I currently use Ahref and it kinda works.
But again, I feel you should manually do your Key Word Difficulty research, just get clues from them SEO tools and get on the SERP page and see for yourself.
I sometimes use the "allintitle" operator and it works for me.
Never ever settle for SEO tools KWs.
My $0.2 though!