HI,
I notice with my Google Account (Also IMAP) that it appears in the Sent "folder" under the Inbox but also has two folders listed in the Google Section of Folders.
I also have a "second" Junk folder here as well.

I think I have acquired these due to Migrating this with my User Account across two computers and several version of the OS (And therefore the Mail App).
Looking in one of those I find it has emails that I sent from my G4
There are only 4 in the first of those and I cannot work out why they are separate. They also look like they relate to the Google Mail account being used on the G4 as well.
I wonder if your Account also has it's own Sent Folder like my Google one has ?

7:56 pm Monday; January 5, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad