folder-hook sort strangeness

[dan radom]

My folders are being sorted using "folder-hook . set sort=threads" in ~/.muttrc.  What 
I've noticed is that new messages that aren't part of a thread are sorted by date 
received.  This morning I received a messsage that became message number 1 in that 
folder for some reason, and I'm not sure why.  Can anyone shed some light as to why 
all messages seem to get sorted by thread then date received except for this one?

Dan