It also depends on whether it's a "proactively curated" account or one which is left to grow without any particular notice being paid to who is following it. For example, I try (when I think about it - my diligence waxes and wanes) to go through the last 20-30 followers I've picked up, and I "block" anyone who looks to be a spammer/completely irrelevant. However, that means that they drop off the follower count. Once you start getting more popular, you'll inevitably attract spammers, so if you don't do anything a higher and higher number of followers will be literally "junk" accounts.