Setting an accurate difficulty level for a puzzle isn't possible.
Everyone thinks differently, has different levels of knowledge, and has different skills. A difficulty level given to a particular puzzle is just a general guide.
If you had to solve 100 'Easy' puzzles and 100 'Hard' puzzles, it would probably take you longer to complete the 100 'Hard' ones, even though you might find some of the 'Hard' puzzles easier than some of the 'Easy' ones.
Generally speaking, 'Easy' puzzles are often easier than 'Medium' puzzles, which are often easier than 'Hard' puzzles, etc., but this is not always the case, and you might find that some puzzles are easier, or harder, than marked.
Perhaps think of 'Easy' as 'Level 1' for that size and puzzle type, 'Medium' as 'Level 2', etc.
As the puzzle sizes increase, the puzzles tend to become harder, so an 'Easy' smaller puzzle tends to be easier than an 'Easy' larger puzzle. Some of the larger 'Easy' puzzles can be quite hard, but they still tend to be easier than the same size 'Hard' puzzles.
Some puzzles might require more advanced techniques as the difficulty level increases, but this doesn't always mean that the puzzle is more difficult.
Different puzzle types also vary in difficulty, e.g., an 'Easy' Light Up puzzle tends to be easier than an 'Easy' Killer Sudoku.
Summary: you can think of the difficulties as Level 1 / Level 2, or Easier / Harder (for that size and puzzle type).
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