My gross salary total does not match with the salary given i

Hi, I recently filed my IT returns assuming that the form 16 details given by my ex-employer, were correct. However when I calculated the total of all my gross monthly salaries, it was lower than the one given in form 16.

Now I have filed my IT based on the information.

Now, my questions are:

1. If the gross salary totals do not match with the amount under the head "Income chargeable under the head 'salaries'(3-5)", can I be sure that the form 16 is wrong?

2. If so, how can I get back the extra amount I have paid? (I paid the tax through online challan 280)

Thanks