Porque isso ocorre por design. O Windows possui um thread de gravador de página modificado (MiModifiedPageWriter) que grava dados modificados de volta no disco.
Aqui está a definição da lista modificada:
The page previously belonged to a working set but was removed. However, the page was modified while it was in use and its current contents haven’t yet been written to disk or remote storage. The PTE still refers to the physical page but is marked invalid and in transition. It must be written to the backing store before the physical page can be reused.)