The clustered index enforces which rows of the cluster belong together on the index pages. If a page has the values 1,3, 4 and 5 on it and value 2 is inserted, that value MUST go on that page. It doesn't have to go in any specific location on the page (it can go at the end after 5) but it must be on that page. If there isn't space on that page, the page must be split.

What is not enforced is the order of the index pages within the file (the page containing {10, 11, 12, 13} can appear earlier in the file than the page that contains {1,2,3,4,5} nor the order of rows on the page.

Yes, I need a blog post on this...