Because cranking the screen resolution up uses more battery and that is what the phone is estimating what will happen but may not be accurate due to real world usage.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Yes, yes thank you, I know that, but it's ridiculous that it estimates only 4-5 minutes less of the battery time. It's ridiculous.