The addresses have been previously used on that iPad and have been retained in memory. They aren't deletable, AFAIK. Maybe you replied to someone on business, etc., and just don't remember that person specifically? Or maybe your wife has?
A long shot on possible removal: Maybe doable if you restore your iPad to factory settings? But that would mean having to restore all content to it later, and might not do the trick for erasing the address memory.
I've restored to factory settings before, but never noted whether that affected the remembered addresses.