[plt-scheme] directory-list strangeness

Here is a very strange transcript...

ls -1 | wc -l
   153409

mzscheme
Welcome to MzScheme version 204.8, Copyright (c) 1995-2003 PLT
 > (length (directory-list))
56848
 > (length (directory-list))
58688
 > (length (directory-list))
61702
 > (exit)

ls -1 | wc -l
   153409

mzscheme
Welcome to MzScheme version 204.8, Copyright (c) 1995-2003 PLT
 > (length (directory-list))
105868
 > (length (directory-list))
117846
 > (length (directory-list))
128088
 > (exit)

ls -1 | wc -l
   153409

Anyone have an idea as to what's wrong here?

-d