从美国经济统计部门下载了分行业的GDP数据，其中有一项说明不懂。请大神帮们解释一下，谢谢！！

1. Chained (2009) dollar series are calculated as the product of the chain-type quantity index and the 2009 current-dollar value of the corresponding series, divided by 100. Because the formula for the chain-type quantity indexes uses weights of more than one period, the corresponding chained-dollar estimates are usually not additive. The value of the "Not allocated by industry" line reflects the difference between the first line and the sum of the most detailed lines, as well as the differences in source data used to estimate GDP by industry and the expenditures measure of real GDP.