Timeline Deliveries represent the total possible number of times someone could have viewed a particular tweet/post. We use the follower count of the original poster to generate the total number of timeline deliveries for each and every hashtagged tweet. (There is no way to calculate how many of these deliveries were actually read, much as there is no way to calculate how many times a magazine ad is viewed, but timeline deliveries give us an idea about how broadly the message has been circulated.)
In contrast, "Reach" is the number of people who have seen any tweet/post regarding a particular hashtag.
So, for example, if two tweets are sent by the same person, then Timeline Deliveries = 2 times Reach.