There are exceptions to 50-50 page splits:
If you insert a huge row that cannot fit on either the page before or the page after the page that requires splitting, two new pages are allocated, one for the huge row and one for the rows that follow it.
If possible, Adaptive Server keeps duplicate values together when it splits pages.
If Adaptive Server detects that all inserts are taking place at the end of the page, due to a increasing key value, the page is not split when it is time to insert a new row that does not fit at the bottom of the page. Instead, a new page is allocated, and the row is placed on the new page.
If Adaptive Server detects that inserts are taking place in order at other locations on the page, the page is split at the insertion point.