Why your payslip disagrees with every salary calculator
The usual culprit is provident fund. Statutorily, PF is 12% of basic wages, but employers may cap the contribution at the ₹15,000 monthly wage ceiling. Many do; many do not; a few let you choose. Nobody mentions it in the offer letter in plain terms.
On a ₹18 lakh CTC with basic at 40%, the difference is ₹57,600 a year against ₹21,600 — a swing of ₹3,000 a month in what reaches your account, on identical CTC. That is a larger gap than most people's expectations of what the calculator should be accurate to, and it is a single switch.
Capping puts more in your bank now; not capping puts more into a tax-advantaged retirement account earning a rate most debt instruments cannot match. Which is better is a genuine question. Not knowing which one applies to you is not.