Scores for sudokus are calculated using the following rules:

Your score is recalculated and displayed each time (and only when) the Check button is pressed.
Points are added to or deducted from your score for each correct or incorrect cell respectively.
Each cell you enter is worth 1 point in an Easy sudoku, 2 points in a Medium sudoku and 3 points in a Hard sudoku. For example, if you enter a 7 into a cell, then you would score 1 point for getting it correct in an Easy sudoku, but you would lose 2 points for getting it wrong in a Medium sudoku.
Once you correctly complete and score points for a cell, you can never score nor lose points for that cell again, even if you were to restart the sudoku.