The correct procedure is to make the 1-liter mix as directed and pour this liquid into a 1-liter bottle. The bottle size is chosen based on a size that reduces the volume of air at the bottle's neck to a minimum.

Next, we pour some of this fluid into the developing tank. The amount we pour must be sufficient to completely cover the film reel.

Now we agitate the film as it develops, 30 seconds continuous at the start, followed by repeated 5 seconds of agitation per minute till time is up. Upon completion of the developing step, we pour this, now used developer back into the 1-liter storage bottle. Likely some of the fluid will be lost, so it is good practice to add a marble or two or three to reduce the air space at the neck of the storage bottle.

What I am telling you is - the capacity of the 1-liter mix is valid if we do not discard used fluid, we return it to the 1- liter storage bottle.

If using the solution, you mixed one-shot (use and discard), you can continue this method until volume is insufficient to cover the reel, or until solution turns dark due to aerial oxidation. This is caused -- the developing agent is a derivative of benzene, initially procured from coal. As the developing agent takes on oxygen, it darkens as it reverts to coal tar (ages and spoils).