'Somebody' is iemand, which is related to the generic noun man 'man'. 'Something' is not related to a generic noun or to an interrogative: it is iets. So iets is a special indefinite. Nevertheless, both iemand and iets carry the prefix ie-, which also shows up in iewers ('somewhere').
Source: Ponelis 1979: 99
Source: Ponelis 1979: 100