That might be because bots opening the emails. Bots can be mimecast URL crawling bots. These bots opens the emails clicks the links in if present for any threat or malware scanning. And hence, you might be seeing these open and such emails are never delivered to prospects. Because these bots don't allow to enter in their mailbox.
Hence your prospects are saying they never received any email.
Check the error codes against such users, there is chance 550 error or other relevant
