The fill factor only applies when an index is created or rebuilt.
SQL server does not try to maintain the fill factor when updating or inserting values.
Quote from BOL:
The fill factor setting applies only when the index is created, or rebuilt. The SQL Server Database Engine does not dynamically keep the specified percentage of empty space in the pages. Trying to maintain the extra space on the data pages would defeat the purpose of fill factor because the Database Engine would have to perform page splits to maintain the percentage of free space specified by the fill factor on each page as data is entered.