This is likely a banned or deleted user, as suggested. The count is difficult to update with this due to this data being stored denormalized and potentially changing unexpectedly. There's code that attempts to handle this, but I think it's wrong. I think we need to just use the fetched count as is in this case, though there are cases that we can't resolve this.
I've adjusted this now to at least deal with the count being incorrect in certain obvious ways (listed as 2, only 1 available). Because of temporary bans and things like that, it may not be possible to have a totally accurate count (that's efficient) in cases with larger following counts.