2 separate emails merged to 1 conversation

We had a situation where an email was sent inbound to our support email. It included 2 parties. 
From - original sender 
CC - 1 person 
To - our support 

Client wrote 1 e-mail which arrived. 
Then client wrote a second email with the same From and CC, but the subject line was different. 

It seems that what happened was the client created 2 separate e-mails and they merged into 1 conversation. Is there any specific use-case where this can happen? Or is this a bug of sorts? I sent this to intercom support, but also curious if I am missing the reason why something like this could have happened.