How it works
Deposits are added at the start of each year for the deposit period, and interest is credited on the whole balance at the close of every year. After the last deposit the account keeps earning at the same rate until it matures twenty-one years from opening.
That six-year tail is shown separately because it is easy to miss and hard to intuit. At the maximum deposit and the current rate, the balance grows by roughly 60% during those years without a further rupee going in — 1.082 to the sixth power, applied to the largest balance the account ever holds.
More detail
The rate is an input rather than a constant. Twenty-one years is longer than the scheme has existed, so treating today's 8.2% as permanent is the optimistic case rather than the expected one.
Nothing about tax is modelled, because there is nothing to model: the deposit is deductible, the interest accrues untaxed and the maturity amount is exempt.
For the same structure without the eligibility rules, see the PPF calculator.
To work backwards from an education cost, use the goal SIP calculator.
To see what that cost will be in twenty-one years, try the inflation calculator.