Dynamic allocation

The kernel generally needs some memory available in order to tell a storage device to write something (even if the something to write is already in place and it's just writing out a dirty file-backed page in cache so that it can free it), so it's got memory reserved for that sort of thing. Presumably, resizing the array gets access to this reserved memory as well, so that it can expand some and swap out more than that so that the memory reservation is restored.