I travel 40 kilometers a day to my first place of work (there and back). However, the tax office only accepts the one-way trip, i.e. 20 kilometers. Why is that?

The tax office applies a commuting allowance for journeys between home and the first place of work. From 2026, this will be € 0.38 for each full kilometer of distance between the home and the place of work.
The distance allowance to be applied is generally limited to € 4,500 per calendar year. This does not apply if you use your own car or a car provided for use to travel between your home and place of work.