Have you deleted any leads from your campaign at any point? You could be seeing stats for older deleted leads as well.
Also, this could happen if you are viewing the analytics for only a specific date range (like a week). It reports the sent and opens within that specific time span.
For example, if someone was sent an email before that week and opens the email in that selected time window, that's when the analytics event will be counted in. If older leads open their emails during this selected time window, they will also be counted in.
Another reason this might happen is if you're viewing this on the Analytics Page (for all campaigns), instead of a single campaign. If you hover on the 'i' information icon next to the open rate, it would show you the open rate by lead and by volume. By lead means that X unique number of people opened the emails. By volume means how many of the emails sent were opened.