Loot at your online sample http://docs.webix.com/samples/61_pivot/03_table_api/02_total_column.html
The value displayed in the footer for “oil (sum)” varies with the number of rows displayed!
The correct value is available only when the top-level items are collapsed; when a top-level row is expanded, the inner-level values are summed up twice, since the sum for these values is already computed in the parent row.
I think this need a fix, or footer is almost useless.