Score estimate is not deterministic. It has a random component.
The purpose of the estimator is, in my view:
- to give OGS feature parity with other Go servers and appease people who like to compare things quantitatively
- to give confused beginners some sense of having something comforting to hold on to in the face of I-don’t-know-what’s-going-on-panic (like a plush toy)
- to give Nova developers an outlet for coding a fancy estimator (before OGS) and now for aspiring programmers to fantasize about writing their own (just better)
For you as a player, I advise you to learn and practice counting. 