Because a day does not have one price.

As the entry price the calculation takes the price at 12:00 on the date you gave. Within that same day the price keeps moving: on 15 June 2025 AAVE stood at 276.83 US dollars at midnight, 271.64 at noon and 268.23 shortly before midnight. That is a good three per cent spread within a single day.

Noon is chosen because it sits in the middle and therefore pulls least in any direction. Your statement carries a different time of day, hence the difference.

If you want it exact, enter your actual entry price in the field on the right. It overrides the looked-up price, and the row is marked as entered by you.