Forwarding email appears like 'On behalf of' to recipients

I have 2 separate users who recently forwarded emails internally but the recipients get an email that has the originator's name as the sender and the user who forwarded email in brackets; as if it was sent on behalf of the originator. The users both swear that they only forwarded the email and neither have Send on Behalf of rights to the originator's email account. This has left the users perplexed and me scratching my head as to why. Can anyone explain this phenomenon to me? Thx