Unlike news articles, which are expected to be short-lived and rarely get updated, a card is expected to live a long time, and get updates as our understanding of the state of the world changes.
An update can be a major update (which notifies followers, and has a short description of the change) or minor update (used for things like typos).
Updates to a card are shown as if they are comments by a user called "Card Updates". This allows you to talk about an update in the comments, and lets you easily see whether a top level comment was written after an update.
The message for an update shows all newly added text in bold. Deleted text is not shown (to improve readability) but you can see the previous text by looking at previous updates.