The term Sachverständiger, expert, is not legally protected in Germany. Anyone insufficiently qualified may call themselves one and offer their services on the market.
So German law built a second tier: öffentlich bestellt und vereidigt, publicly appointed and sworn. Only specialists whose knowledge lies considerably above average are appointed, after an exacting examination procedure, and their work then stands under the permanent supervision of the appointing body — the chambers of crafts, of industry and commerce, of architects, of engineers.
One line from the guidance issued by the Regensburg chamber states the principle exactly: performing your profession properly is not yet sufficient proof of special expertise.
Doing the job well is the floor, not the qualification.