I have notice 1 thing that may be incorrect. According to the SRD, Synthesist states the following
While fused with his eidolon, the synthesist uses the eidolon’s physical ability scores (Strength, Dexterity, and Constitution), but retains his own mental ability scores (Intelligence, Wisdom, and Charisma). The synthesist gains the eidolon’s hit points as temporary hit points. When these hit points reach 0, the eidolon is sent back to its home plane. The synthesist uses the eidolon’s base attack bonus, and gains the eidolon’s armor and natural armor bonuses and modifiers to ability scores. The synthesist also gains access to the eidolon’s special abilities and the eidolon’s evolutions. The synthesist is still limited to the eidolon’s maximum number of natural attacks. The eidolon has no skills or feats of its own. The eidolon must be at least the same size as the synthesist.
According to this while fused you only get you eidolons BAB and you do not add any other totals to it. while you totals from all your classes is +12, the second you fuse you are dropped down to +6 (assumably beacause you eidolon is not as ex as you are).So the +12 total on your fused form should be +6