Twitter has a very specific formula for how they calculate characters. This formula can affect longer posts, particularly if they include links.
The new Twitter calculation formula does allow for longer posts, but only if they include t.co links, rather than your own included links. Any links that are placed into a Twitter post are automatically counted as 23 characters, regardless of the actual character count in the post. If you include more than one link, each of those links will be counted as 23 characters.