Why is this method wrong?

While finding the TSA of the solid, why can't just just do this-

CSA of Cone ➕ TSA of Cylinder ➖ Area of Base

I got the wrong answer doing this! Need help with explanation.

While finding the TSA of the solid, why can't just just do this-

CSA of Cone ➕ TSA of Cylinder ➖ Area of Base

I got the wrong answer doing this! Need help with explanation.