The decimal format in the frontend is 4 decimals, the format in invoices is 2 decimals by default. Both formats are defined in the ispconfig language files.
If you use a custom pdf invoice template, then it might be that it needs to be updated to use the 2 digit dezimal format for the pdf file like it is used in the templates that are delivered with the current billing module version 1.3
If you like to change one of the decimal formats, go to System > Languages, select your languge and click on the global file (e.g. en.lng).
number_format_decimals is the one that is used in the ISPConfig interface and number_format_decimals_client is used in the pdf invoices and in the client dashboard plugins.