Why? Well, because people are continually trying to sort out things they don't yet have sorted out, and people then make these diagrams
Layer one is easy: Classical Qabbalah attributes no planet past Saturn to any part of the Tree of Life. Pluto, in this sense, is attributed to NO part of the Tree.
Layer two gets complicated: People have different views about Pluto and, in particularly, most astrological things written from Pluto's discovery in 1930 until well into the 1960s will have a very distorted view of astrological Pluto. Additionally, almost nobody alive has more than an intellectual idea about either Da'ath or Kether. So, when different people bring different definitions or understandings to two things that they are trying correlate, they end up coming to different conclusions.
For example, somebody might have attributed Pluto to Da'ath because they think that Da'ath is catastrophic and dismantling. IMO that's mistaking some phenomena - how some people react to Da'ath - for Da'ath itself. Or they might see Da'ath as a gateway to the underworld and take a mythological equation. Etc.
There might also be a historical reason. There was a trend, about the time Pluto was discovered, to already attribute Uranus to Hakh'mah and Neptune to Kether. Then, when Pluto was discovered, they needed a new box to fill - and Da'ath provided it.
I personally equate Pluto to Kether, but I don't teach
this attribution because Qabbalah does not have this attribution. It's a personal invention. I'm entirely settled in my reasons for this and it serves me in practice. Three things about Pluto might help explain my view: (1)Pluto, at root, is that particular transformation that occurs one one truly gets trans-form, i.e., outside of any "extensions" into particular pattern. (2) Pluto, in practice, especially when interacting with other planets, represents complete disinterest or reaction against anything arbitrary, artificial, forcing into molds or patterns that are not intrinsic to what a thing is. (3) Pluto is inherently solitary, "outside the herd," outside the pattern.