Open rates are calculated based on the recipients of the campaign who are not using an email client that blocks open tracking.
Last year, Apple launched iOS15 which gave Apple users the option to disable open tracking in the Mail app. To have the most accurate representation of your open rate data, recipients who have chosen to turn off open tracking are automatically excluded from the open rate calculation of your campaigns.
For example, if a campaign is sent to 1000 recipients but 400 recipients are using an email client that has allowed them to block open tracking, those 400 recipients are excluded from the open rate percentage calculation. The open rate percentage will be calculated based only on the 600 recipients who have open tracking enabled. If 25% of those 600 recipients open your email, your estimated open rate will be 25%.
All subscribers who have opted in to block open tracking are automatically added to a segment in your Hive account called "Subscribers Who've Disabled Tracking". To learn more about how to incorporate this segment into your marketing strategy, check out this article; What is the "Subscribers Who Have Tracking Disabled"