I wonder how MC determines the sequence numbers. In a static playlist it is the list order, but a smartlist doesn't have a static order.
EDIT
It seems to be the sequence order in the same list when the number of tracks is not limited. However, probably it would be useful to control how that big list is sorted with a few additional sort rules.
EDIT 2
For example, "Top 401-500":
[Media Type]=[Audio] [Number Plays]=>0 ~sort=[Number Plays]-d,[Last Played]-d,[Artist],[Name] ~n=500 ~seq ~sort=[Sequence]-d ~n=100 ~sort=[Sequence]
The sequence number column in the list view is indeed nice.