Old Scratch first begins with Bone Chant, yes? When you promote him to Bokor he gets Jobu's Kiss, but no upgrade to the summon is made. However, once you promote Old Scratch to Houngan, the Skeletal Pirate summon changes to a Skeletal Corsair summon, and Jobu's Kiss stays the same. Does this make sense to you?
As I understand it, the Summoned Minion statistics depend on how high a level you are and how high your Will is. Also, if you cast some powers that add to your Will (ie Big Juju, Juju, Calm the Troops, etc.) before you cast the Summons, your Summoned Character will be stronger. My impression is that just going up levels will increase the Summoned Character statistics but I wouldn't expect to see much if any improvement until the caster has gone up at least 5 levels and possibly more.