Instant Audiences uses the same mechanism that Klaviyo uses to identify users clicking on emails from Klaviyo. This is why Audiences started sending website traffic information regarding this email to Klaviyo. Users already known are not added to the net new user list but as we sent events for this profile, it was added to the "All" list.
For customers that we identify this way, we persistently track their website traffic using Instant’s proprietary identification technology that leverages a combination of server-side, client-side, cross device and advanced browser identification techniques.
Recent updates from Apple/iOS and Chrome/Firefox, limit Klaviyo's tracking ability to only 7 days (therefore only identifying 10-15% of abandoners). If a shopper then comes back after 7 days, Klaviyo has lost the ability to collect signals on that customer, meaning no abandonment signals are collected.
However, with Audiences, Klaviyo will continue to receive user signals and events indefinitely, even after 7 days on Apple/iOS/Firefox/Chrome or when they block/delete browser cache/cookies. This greatly increases the amount of abandonment events that Klaviyo has, resulting in increased abandonment revenue.