I have recently been discussing the creation and grading of sudoku puzzles with a member of my Sudoku Puzzles Facebook group.
I make my own puzzles using a C program, which I wrote myself in 2012. This creates a valid solution then randomly removes as many clues as possible, whilst still retaining exactly one answer. I then grade each puzzle using Hodoku and/or Andrew Stuart's grader and solver (see link below puzzle).
I find that this method produces puzzles with a range of difficulties from easy through to extreme. None of my puzzles have any surplus digits.
The member mentioned above asked me to grade three puzzles for him so I submitted the first one to Andrew Stuart's grader and solver (see link below puzzle), which said it was very easy. It also had several spare digits i.e. clues which could be removed to leave a slightly more difficult puzzle, which still had exactly one solution. I have seen this from time to time in sudoku books but personally I don't like it.
I removed as many of the spare digits as possible, which created the diabolical puzzle below:
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