If you’re looking at a contributor or tweet in your report that has only generated a single impression, even though you know the account has hundreds or thousands of followers, then the tweet is likely an @reply. On Twitter, replies are only sent to users who follow both of the Twitter accounts involved in the conversation. So 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 (i.e. people with the context to understand the @reply). Basically, any tweet that begins with a username (like "@tweetreachapp I love your analytics!"), will be considered a reply and will only be delivered to common followers. Due to the recent 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.
Twitter's goal with this is to keep the conversation going without bombarding the Twitter feeds of users who don’t know both the people talking with posts that have no context and are thus pretty irrelevant. If you're interested in learned more, read about how Twitter thinks about @replies here.
If you want to make sure a tweet goes out to all of your followers, then don't start it with @username. Check out this page from Twitter on where your tweets appear. 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.