First of all, it’s crucial to understand the hard and soft bounces in general, to be able to fully understand reports. If you don’t know what bounces are, please read article – Soft vs Hard bounces.
Let’s assume it’s a report of a campaign you previously sent
There are 22814 sent emails. The ‘Bounced’ section tells you that 6,91% (1576) of your emails bounced (not delivered). So, the remaining email addresses that are valid are 21.238.
For the reports, the number 21238 is used for mathematics. That means that the open rates are being counted using the number 21238, not 22814. It’s intentional – the user should focus on the real open rates for valid email addresses.