It could be the fact that systables
stores entries for each table, view or alias, in fact a large number of objects that may not necessarily be included within a tablespace.
You could confirm this by only listing those where type = 'T'
(or some other combination of the allowed values).
If you select count(*)
from systables
(for a given tablespace) and group it by type
, you may find it reasonably easy to assign some of those types to the tablespace.