Thanks! I agree that signing up in order to enable reading lists (or other apps features, I am not very familiar with their capabilities) can explain why we see about 10% of creations happening that way.
The way I identify "apps" in the analysis is simplistic, it only looks at the status of the
isApi flag, because my exploratory analysis suggested that apps use the API for account creation (while the mobile and desktop sites use a different process). This means that if the API is used for account creation, e.g. to create several accounts for an event, this will also fall into that category. When I then split it by looking at the
os_family part of the user-agent string, those API account creations are neither iOS nor Android and fall into that "other" category.
So it's not the case that those accounts are created in apps, but just look that way because of how the categorization works. I'll make a note to update the page to clarify that as I can see that being very confusing.