When turning the Riemann sum into the definite integral, why is the definite integral integral from 1 to 2 (x+1)²dx and not integral from 1 to 2 (x)²dx?

When turning the Riemann sum into the definite integral why is the definite integral integral from 1 to 2 x1dx and not integral from 1 to 2 xdx class=