ata eva nityadravyavṛttayo 'ntyā ityubhayorupādānam / tatra parasparamatyantavyāvṛttabuddhihetutvāt svāśrayamanyato viśeṣayantīti viśeṣā ucyante //
This is precisely why both terms - "ultimate" and "subsisting in eternal substances" - are used. These are called "viśeṣas" because they cause the cognition of absolute mutual exclusion and thus distinguish their substrata from everything else.