We've recently updated the way sub-categories are displayed on category pages. Sub-categories were previously shown as a simple list, but are now arranged in columns for better readability.
Up to 16 subcategories are shown initially (4 rows of 4 columns). For those categories with more than 16 subcategories, a simple "More categories" link is included so that customers can view/hide the full list.
Sibling categories are still shown, although since sibling links tend to be of secondary importance, by default they are still formatted in the old "list" style which saves space on the page. We use the term "siblings" to describe the categories that at the same level of the category tree as the currently viewed category.
This is purely a user interface change with very little change to the underlying semantic mark-up. SEO should not be adversely affected.
Both of these groups of sub-category and sibling links on the category page can turned off through your admin's store settings if they are not required or relevant to your store.
