The icons that we see in the password list are fetched from the website I believe. Most of the websites when a favicon is available, it gets them, but in many cases the 1password is unable to fetch the icon and shows the first two letters of the website. May be it is not actually getting the icons from favicons, but rather from some other source, cause in many cases I see the websites clearly have their favicons but 1password fails to fetch them. To me the biggest irony is that 1password can not fetch the favicon of the 1password forum itself!
PS; I am aware that the icons could be changed manually. But that is not the point.
Exhibit A: In case the image does not load: Here is Imgur link
The same is the case for browser extension too.