It looks like you are trying to use a form button to get the inputted values from the user. This will not give you a realtime calc because data needs to be written to the sheet before the math column will be able to calculate anything. In a form, nothing is written to the sheet until you click on submit.
The calculated value you are seeing is coming from the detail row of the first row you are viewing that contains the form button.
Do you intend to submit the form once they enter data, or are you only using it as popup screen for an instant calculation without any intention of saving the entered values?