How is TDS on salary calculated?
Your employer estimates your total tax liability for the financial year based on your annual salary and the tax regime you've chosen (or the default new regime, if you haven't declared otherwise). Whatever tax hasn't already been deducted is spread evenly across the remaining salary months, so you don't get one large deduction at year-end.
Why does my TDS change during the year?
TDS is recalculated whenever your salary changes (a hike, bonus, or new joiner mid-year) or when you submit fresh investment declarations and proofs. Employers typically ask for proof of old-regime deductions around January-February, after which TDS is adjusted for the remaining months.
What if too much or too little TDS was deducted?
If your actual tax liability turns out lower than the TDS deducted, you can claim a refund when filing your income tax return. If it's higher — for example, you have other income not reported to your employer — you'll need to pay the balance as self-assessment tax before filing.