Yeah, you have a bundle conflict somewhere, meaning you have two applications with the same creator code with different icons (presumably one is the newer version with the icon you don’t like) so the Finder is picking one randomly and using it for both instances.
As you surmise, one fix is to find the rogue installer hiding on your HD and delete it.
The other is to do a manual override … find the older installer, open it with ResEdit, and copy the ICN# or (if you’re using a color display) icl8 resource image you like to the Clipboard. Then paste it into the icon box in the Finder’s Get Info window.