INsert vs Update

INsert vs Update

Hello, I am trying to do an update record on a LEAD. It seems as THOUGH I DON't Even need to use update, just insert command on the same EMAIL address record just updates the record. 

Is this by design, a bug, or is Insert different then Update, when a record with the same email exists?

https://crm.zoho.com/crm/private/xml/Leads/insertRecords
https://crm.zoho.com/crm/private/xml/Leads/updateRecords