Why does president obama address?

seiu and afl-cio members as "brothas and sistas" during his speeches yet in his other speeches addresses americans as just "folks". i watched his speech to the labor union today and he sounded like a preacher giving a powerful sermon. i went back to re-view his speech to seiu earlier this year and lo and behold, the same affectation to his voice and speech pattern. does anyone have any comments and has anyone else ever noticed this?