Ah, so you were using an RSVP block. RSVP blocks use link click tracking like other links you might manually insert into your email. So, if your contacts are forwarding the email, then it's going to result in the forwardees affecting their selections since those links are uniquely coded to each contact upon send-out. If your contacts are running any programs that check for malware by clicking every link, that will also affect the reporting.
Overall, if you're wanting to get people to sign up for an event, I'd recommend using the actual event tool and registration form, not the RSVP block. The RSVP block is more for quick-selection and interest gauging.
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