What does "net cost" mean here?
Both paths start from the same cash position — the down payment and buying costs you'd otherwise spend on a home. Buying spends that money on the house and offsets it against the home's equity (its value minus any remaining loan) at the end of the horizon. Renting keeps that same money invested instead, growing at your assumed return, offset against total rent paid. Whichever path has the lower net cost comes out ahead financially over that specific period — it isn't a verdict on which is "better" in every sense.
Why does the time horizon matter so much?
Buying has large upfront costs (down payment, stamp duty, registration) that take years to "pay off" through equity growth and avoided rent increases. Over a short horizon — say, 2-3 years — renting is very often cheaper, since you haven't had time to build much equity or benefit from appreciation. Over 10-15+ years, buying usually closes the gap or comes out ahead, especially in markets with steady appreciation.
What this calculator doesn't account for
It ignores income tax benefits on home loan interest and principal (Sections 24(b) and 80C), the emotional/stability value of owning, liquidity differences between a house and a mutual fund, and city-specific stamp duty rates — all of which can shift the numbers for your specific situation. Treat the result as a financial starting point, not the final word.