This occurs when: 

Constituent #1 has an email address and a preferred address.
Constituent #2 has a preferred address.

The constituents are merged, with #1 being the source constituent. Constituent #2 will now have two addresses listed on the addresses tab, a preferred and an alternate.

If we open the alternate address, the email will be listed there. If necessary, delete the email address listed here and enter it on the Bio 1 tab.