Why does the contract-note amount differ from the app estimate?

The pre-trade estimate shown in the order window is calculated based on the price and quantity you entered before execution, using current rates for STT, exchange charges, and GST at that moment. The contract note reflects the actual executed price and quantity, which can differ from what you initially entered, especially with a market order that fills at a slightly different price, or an order that gets partially filled across multiple price points, each contributing its own charge calculation. Small rounding differences across multiple partial fills, or a same-day rate change in one of the statutory charges (rare, but possible), can also create a marginal gap between estimate and final. You treat the estimate purely as a planning figure and the contract note as the source of truth, and if the difference is more than a small rounding gap, you go through the actual fills in the order book to understand exactly which part of the trade differed from what you expected.