Over time, the effects of randomly distributed data modifications on a set of tables tends to produce data pages and index pages that average approximately 75 percent full. The major factors are:
When you insert a row that needs to be placed on a page of an allpages-locked table with a clustered index, and there is no room on the page for that row, the page is split, leaving two pages that are about 50 percent full.
When you delete rows from heaps or from tables with clustered indexes, the space used on the page decreases. You can have pages that contain very few rows or even a single row.
After some deletes or page splits have occurred, inserting rows into tables with clustered indexes tends to fill up pages that have been split or pages where rows have been deleted.
Page splits also take place when rows need to be inserted into full index pages, so index pages also tend to average approximately 75% full, unless you drop and recreate them periodically.