When does Windows decide to pull out pages from Working Set?

When does Windows decide to pull out pages from Working Set?

I am looking at the distribution of memory using the RAMMap tool by
SysInternals. The distribution (see image below) shows
1.7 GB in Free (Zeroed) memory list
1.2 GB in Working Set list
550 MB in Standby list
125 MB in Modified list

I understand that when a process requires memory, Windows searches for
memory in the lists in the order Free (Zeroed) -> Standby -> Modified.
Hence, as long as we have Free page frames, everything should either be in
the Working Set list or Free list.
Is this assumption right?
If so, why do we see 125 MB of pages in the Modified list?
In general, when does Windows decide to pull out pages from Working set
list to Modified/Standby list even when there is Free memory left?
I am using Windows 7, with 4.0 GB of installed RAM, 3.5 GB being usable.