This is a Riemann Integral calculating the height of rectangle with the value of the function for their right side. That's why it overestimates. If you would have taken the left side you would have k going from 0 to n-1 instead of 1 to n and in that case it would underestimates.

Although, when you calculate the limit it goes to the same value. You thus only overestimates when there is no limit.