This is because not every Klaviyo profile will be a Shopify customer. There are other ways in which profiles can enter Klaviyo directly without first passing through Shopify. For example, through a Klaviyo sign-up form (assuming the user has not started a checkout or made a purchase), they would be added to a Klaviyo list without becoming a Shopify customer.
The easiest way to identify these profiles is to export a .csv file of all your Shopify customers, import them into a new Klaviyo list. Next, create a segment, defined as: if someone is in or not in a list > is not in list X using the new list name. That will return all profiles who are in your Klaviyo account but not your Shopify list (which contains all Shopify customers).
This is because not every Klaviyo profile will be a Shopify customer. There are other ways in which profiles can enter Klaviyo directly without first passing through Shopify. For example, through a Klaviyo sign-up form (assuming the user has not started a checkout or made a purchase), they would be added to a Klaviyo list without becoming a Shopify customer.
The easiest way to identify these profiles is to export a .csv file of all your Shopify customers, import them into a new Klaviyo list. Next, create a segment, defined as: if someone is in or not in a list > is not in list X using the new list name. That will return all profiles who are in your Klaviyo account but not your Shopify list (which contains all Shopify customers).