Concrete mixes have become more complicated over time. Flyash has been around for a while, GU/L is relatively new and seems to set faster, often requiring retarders. Many different water-reducing additives are available. Air entraining agents tend to reduce strength. Fibers or steel pins added to the mix can improve crack resistance.

Batch plants will design mixes so some water can be added on site to improve workability. If you don't add water, the concrete will likely exceed spec.

A slump test is only one factor if many that impact concrete strength.