Individual tweets within conversations on Twitter are now being ranked on mobile devices. Based on a few factors, individual replies will appear higher or lower. This same setup has been on the web for a while. You can expand a tweet to see the full conversation as usual.
Another change to the mobile version of Twitter is that tweets include a reply counter, giving you another way to see which posts are potentially the most influential. Twitter previously included counters for retweets and likes.
Twitter's ranking for conversations favors content from accounts you follow and from the original poster.