Composing an email, then receiving new email during compose, then sending results in response to new email

[mdillondc]

• [email protected] sends me an email. I am in the process of composing a reply.
• While I compose the reply, [email protected] sends me an email unbeknownst to me
• I finish composing my email to john and send it, only to find that the email i'm actually replying to is the one from jane.

Why does this happen?