Not a mistake; a balancing factor. A Favored Soul needs two stats high for the spellcasting oomph of a Sorceror - as the Favored Soul gets all good saves, 3/4ths BAB & several nifties, and more spells known than the Sorceror, while the Sorceror gets..... a Familair, one good save, and 1/2 bab.
Hmm... I think the Favored Soul is ahead in a great many respects....