When Emma, Cleo and Rikki became sirènes they were together and the powers divisé, split into three so each mermaid had a power. When charlotte became a mermaid she was alone so the powers didn't split, and she also got the additional powers (fire and wind).
Well, perhaps because she was alone when she bacame a mermaid! Emma, Cleo and Rikki were all together, each mermaid a power, but charlotte was alone, so she's got all three powers, my guess...
