Could you send through an example? To hopefully shed a little bit of there are a few things that can cause the discrepancy.
In the 1 month or any historical view, we plot close of day prices and the performance is calculated based on the first data point of the series (the first one shown in the chart) against the latest one, which is the close of day of the last trading day or it can be the latest data point from during trading hours.
During trading hours the latest data point can vary depending on platform as different platforms refresh the latest data point more or less frequently than others. It’s important to check what that latest data point is compared to any other provider which can cause some discrepancies during trading hours.
Another reason you might find a discrepancy can be the handling of the close of day price. Different brokers handle end of day prices differently. Currently, we take the last price given to us by our data provider for the day whereas others might update the price after the closing auction period later in the day.
Hope this helps but do let me know if you have more questions