That is because taxing previously sheltered income yields less revenue when rates are lower.
这是因为先前的税收政策对收入免税，在税率更低时税收收入就更少了。
英语百科
Current yield
（重定向自Income yield）
The current yield, interest yield, income yield, flat yield, market yield, mark to market yield or running yield is a financial term used in reference to bonds and other fixed-interest securities such as gilts. It is the ratio of the annual interest payment and the bond's current clean price: