Yes, I understand that.
However, everyone’s U.S. Passport lists the “Country of Birth” and if Obama’s Passport stated that he was born in Kenya, someone along the line would have noticed that fact.
Sure, someone may have noticed it, if he were just some lowly American peon waiting in the Customs & Immigration line, but he's a bigtime VIP US Senator. Those people go through the diplomatic express line, and are never given anything but scant scrutiny.
I highly doubt that any immigration agent ever gave his US passport anything more than a cursory glance before stamping it.