The systabstats and sysstatistics tables store statistics for all tables, indexes, and any unindexed columns for which you have explicitly created statistics. In general terms:
systabstats stores information about the table or index as an object, that is, the size, number of rows, and so forth.
It is updated by query processing, data definition language, and update statistics commands.
sysstatistics stores information about the values in a specific column.
It is updated by data definition language and update statistics commands.
For more information, see “Effects of SQL commands on statistics”.