Others on the forum will give you a more informative answer but this will give you something to ponder...
Think of the pipes as the end of the complete system, which begins with air intake. They all--air intake, fuel supply (carb or efi), engine, and exhaust need to be "harmonized" to get maximum power. The pipes create backpressure, which has a direct effect on fuel mixture. Changing the flow at one end and not the other will throw the entire system out of balance. This is true for either end of the system. So, you've changed the fuel mixture by changing the backpressure in the exhaust. Now you need to make the appropriate adjustments at the intake and then adjust the efi accordingly.
Ain't it fun?!
Roger