Why does the fizz strength of the beverage in the sodastream ULTIMATE Multi-drink™ change even when I use the same Fizz Level?

The best fizz outcome can be reached when the water in the water tank is cold. When the water is not cold, the fizz level is lower. 
If the water is not cold enough, you can either replace the water with cold water or add a few ice cubes. Best practice would be to have additional Water Tank on hand, for best cold-water management.