Its because Barack Obama was born long before most hospitals were required to have computers to keep track of your birth certificate. Since Barack Obama has shown only an replica of his original birth certificate, it maybe a possibility that Barack Obama had a new one made, if he actually shows the real one 2 things will happen.

1.) Barack Obama is an US Citizen and the birth certificate will be the original copy.

2.) Barack Obama is not an US Citizen and that this is the biggest idenity fraud in US History, which will more then likely result in an major upset for the Democrats, might end up losing the Senate and more seats in Congress with Barack Obama being Impeached for violating the requirements for President.

I hope this helps =)