Why are there two blocks of Greek characters in the Unicode Standard?
The layout of the Greek script in the Unicode Standard is an artifact of the history of Unicode and of ISO/IEC 10646. The Unicode Standard started out with just the Greek block (U+0370..U+03FF), with Greek characters laid out in compatibility with the modern Greek monotonic standard, ISO/IEC 8859-7, and with additions for some Coptic, ancient Greek, and Greek symbol letters. When the Unicode Standard had the repertoire from drafts of ISO/IEC 10646 merged in, as part of the standards compromise which resulted in the synchronization of the Unicode Standard and 10646, the Unicode Standard acquired a collection of pre-composed Greek characters which were intended for polytonic Greek usage. Those had to be placed somewhere, and a “compatibility” block was created at U+1F00..U+1FFF to accommodate them.