Check the annual tax in Form 16 Part B against its taxable income. See which regime, taxable income, or late declaration shape best explains the employer figure.
Why does this page ask for Part B and not Part A first?
Because Part B carries the annual tax computation and this page checks that computation. Part A is the quarterly certificate of what was deposited. Deposit timing, TAN mistakes or challan problems can make Part A wrong while Part B is still arithmetically right, and the reverse is also possible.
Why can this page not judge a second employer or other income?
Because one Form 16 is then not the whole year. A later employer may have folded the first salary in, or may not. Other income may have been declared to payroll, or may have been handled in advance tax instead. Once the year is bigger than this one Part B, the page can still price the taxable income shown on it, but it cannot say the form is the whole truth of the year.
What does it mean when the tax deducted points to another regime?
It means the easiest explanation of the figure is that payroll computed tax under the other regime from the one printed or selected. That is the first assumption to check before arguing about deductions. A regime flip can move the answer by far more than any single 80C or 80D line.
What does it mean when the gap looks like one quarter of the year?
It often means the final deduction or exemption was applied only for the last quarter. Payroll is allowed to adjust late declarations, but a late HRA or 80C proof can still leave one quarter carrying the old tax reading. This page does not prove that happened. It tells you when the shape matches it closely enough to ask the question.
Why do you work from taxable income instead of from gross salary alone?
Because Form 16 Part B already states taxable income, and that is the exact line the annual tax is computed from after exemptions and deductions. Gross salary alone would need this page to rebuild every deduction the form already names. It does rebuild them as a consistency check, but the tax is judged on the taxable-income line itself.