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Mechanical wear I guess. You will notice the difference when you get there when comparing to new sheet.
My H3 sheet is now 7 weeks old, and I actually feel like its in its prime condition. Serves and topspin loops are very effective.
I'm actually looking for a reason to change rubber, because I have all these other rubbers I have stored up and need to use. But objectively I can't actually justify the need to change my H3 sheet yet. It's still near perfect playing condition.