Slabs are marginal, and the fear about them is misplaced
A great many people believe that crossing into a higher slab taxes their whole income at the higher rate, and decline additional work because of it. That is not how it works. Each band is taxed at its own rate, and only the income inside the higher band pays the higher rate.
Earning ₹1 more than a threshold means paying the higher rate on that ₹1. Take-home never falls as income rises. The one genuine exception is a rebate that phases out at a threshold, where income just above the line can leave you slightly worse off — which is why the calculator shows the figure either side.