If you utilize Sprout Social's Sent Message reporting, you may notice that Sprout's Reach statistic is normally much higher than Twitter's "Impressions" number. This is due to how our platform gathers and calculates Tweet performance.
Sprout calculates a Tweet's reach by the number of followers you have when you post the message, plus the number of followers of each person who retweets your Tweet. Because it's likely that your Tweet won't be seen by all of those people, we consider this the potential maximum reach.
Twitter, on the other hand, knows exactly when a Tweet has been seen in a user's timeline, so they can give a more accurate Impressions number. However, they can only determine if a Tweet has been viewed when it has been served by an official Twitter platform (their website and official mobile apps, for example). So their number can be considered the very minimum number of times it was seen.
Utilizing both the Sent Messages report in Sprout Social and Twitter's own analytics together can give you a good idea of how many times your Tweet has been seen.