Why do estimated charges differ from final charges?

The estimate shown before you place an order is based on the price and quantity you’ve entered, but the final charge depends on what actually got executed, which can differ in a few specific ways. If your order fills at a slightly different price than expected (common with market orders or in a fast-moving stock), the percentage-based charges like STT and transaction fees shift with it. If the order fills in multiple parts at different prices, each fill can carry its own charge calculation, and rounding across several small fills can create a marginal difference from a single estimated total. Auto square-off, if triggered, can also add a fee that wasn’t part of the original estimate. You treat the pre-trade estimate as a ballpark and the contract note as the actual, and if the gap between them looks unusually large rather than a rounding difference, you ask the broker for an itemised explanation.