Both seem like possibilities in terms of what the endpoint might filter on. Are these two items actually referring to the same thing? Could they ever not match? If so, which item does this endpoint filter based on?
Search should be running against the value in the email contact property. This is the field that holds the contact’s current email address, while you may see historical or conflicting values in the identity profiles (this usually happens when an email address is updated, or two contacts with different email addresses are manually merged).
Identity profiles are basically how we track and associate analytics data with contacts. Basically, each person visiting your hubspot hosted content is associated with a hubspot cookie (UTK) which we use to track their visits and actions on your site, and to associate those visits to the right contact when a visitor converts. You often see multiple identity profiles because a contact converted in your HubSpot account twice from different computers or browsers (e.g. they filled out one form from their work computer, but later browsed around your website and converted on another form on their phone). Another common reason for multiple identity profiles is contact merging.
