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The value for Settlement Days has no effect on the bond npv, since the npv is calculated wrt the valuation date rather than the settlement date.
It only affects extra results such as the clean/dirty price and the settlement value.
Click on download to download an xml file that contains an Excel formula that demonstrates this issue.
You may then reproduce the issue in spreadsheet by clicking on Go -> Load Excel Formula from Text File or one of its variations and choose the xml file downloaded in the step above.