In some cases, you may encounter a tweet that has generated 1 potential impression even though the account that sent the tweet has many followers. If you come across this, the most likely reason is because the tweet was a reply.
On Twitter, standard replies are only sent to users who follow both the Twitter accounts involved in the conversation. So even if an account has thousands of followers, a reply will only appear to users who follow both the original tweeter and the person replying. In some cases, that means that tweet only generates 1 potential impression - it shows up in the timeline of the original tweeter only. If you want to make sure a tweet goes out to all of your followers, then don't start it with @username. Or use a common workaround, like adding a period before @username in your tweet.
In some cases, a reply may generate more than 1 impression through amplification. A reply can be amplified the same way a standard tweet can, so if a reply is later replied to, retweeted or quoted (retweeted with a comment), then it will generate 1 impression plus additional impressions from the reply, retweet or quote.
Finally, if you reply to yourself (meaning the tweet starts with your own @username), it will appear to all of your followers, so while that looks like a reply, it functions like a standard tweet and will generate full potential impressions equal to the number of your followers at the time of the tweet.