You are either lifting too early, or too late. Osu combines both your press and the release into 1 timing judgement (at least in score v1). If you are too early or late at releasing the note, even a perfectly timed press will be reduced to a lower judgement like this.
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Look at your timing bar at the bottom during a replay. It's an extremely useful tool! There you can see if you're lifting/starting a freeze note too early or too late. Most importantly, click and lift to the beat! It's a rhythm game after all