There might be some instances in a report where a tweet has generated only a single impression even though the account has many followers. If you come across this, the most likely reason is because the tweet was a reply.
On Twitter, replies are only sent to users who follow both of the Twitter accounts involved in the conversation. That said, even if an account has thousands of followers, replying to a tweet will only appear to users who follow both the original tweeter and the person replying. Due to changes in Twitter's API, we can no longer fetch the actual unique IDs for the intersecting followers who received a reply, so we will assign all @replies a single impression in our exposure calculation.
If you want to make sure a tweet goes out to all of your followers, then don't start it with @username. You can learn more about where tweets appear from Twitter here. And to be safe, do not click the reply button and then add text before the username, such as a period or other punctuation. Even an altered @reply may still only be delivered to common followers.