It seems like Writer (as well as all other LibreOffice applications) default to centimetres when using metric units (e.g., in Options > Settings > Measurement unit). However, all document-related standards and conventions are all defined and measured in millimetres, not centimetres. For example, all the ISO paper sizes (e.g., A4) are defined in millimetres, and all countries around the world that use metric units for paper sizes measure them in the same.
You can change this under Tools > Options > Writer (other modules) > General > Measurement Units. Controls with units also allow to insert a value with a different unit like "42 mm" for "Tab Stops" underneath the mentioned unit selection changes the value into 4.2cm on leave (eg. per tab). Whether the default for metric values should be millimeter or centimeter seems to be a personal preference. But I never discussed the dimension of A4 in mm, it's always cm. And there are some other tickets around the topic so I'll make your request a duplicate. *** This bug has been marked as a duplicate of bug 72662 ***