Ah, found it! From the 'In' tag documentation:
'orphan' -- The desired minimum number of objects to be
displayed. The default value for this
parameter is 3.
That's why 7=5+2 doesn't work as you expect, but 8=5+3 does!
Try:
<!--#in "objectValues(['Folder', 'DTML Document'])" sort=id reverse size=5
orphan=0>