could i have made a mistake at the time
Yes.
& not completely downloaded bitcoin core causing the funds to slip to another address?
No.
Would i have had multiple wallet addresses in my bitcoin core address that i should be still able to access?
I would assume Armory's root key restores all addresses it ever used.
im certain this is my old wallet showing this transfer that i never made
Which address is your in your wallet: 1A56jABzc62Nchh7JSkU9zW9H2BDXHvmiG or 1Cvoe4whXCKB7hL9KJEZHvT22Hdiitz26H?