[Someone]

Many of such warts in Unicode are for allowing round-tripping with 8-bit character encodings. I suspect that’s the case here, too. https://en.wikipedia.org/wiki/ISO/IEC_8859-7 has them, too.

That doesn’t explain why Unicode seems to have 27 (!) different “sigma” code points, though (https://en.wikipedia.org/wiki/Sigma#Character_encoding)