How do they achieve consistency in civil laws systems?
In the US if say a district court in California and a district court in Oregon adopt incompatible interpretations of a federal law, someone will appeal to the appeals court that covers both California and Oregon, that court will interpret that law and then that is binding in all that states under that appeals court (Alaska, Arizona, California, Hawaii, Idaho, Montana, Nevada, Oregon, and Washington).
If some other appeals court in some other region goes a different way, it can go to the Supreme Court which makes an interpretation for the whole country.
We eventually reach consistency even if Congress is unwilling or unable to revisit the law.