Is this all happening inside a native form?
My suspicion would be that you are seeing the result of the calculation in the first row of your target table.
One thing to be aware of is that it’s not possible to show the results of computed columns inside native forms (if the computations use the values to be submitted). This is because form data is held in memory and only written to the underlying table after the form is submitted.
To be able to show real time calculations inside forms, you need to use Custom Forms.