Fast and slow digestion explain the two curves
Whey and casein are both dairy proteins, but they behave very differently in the gut. Whey is soluble and empties from the stomach quickly, producing a sharp rise in blood amino acids that peaks around 60 to 90 minutes and largely clears within a few hours. Casein clots in the acidic stomach, forming a gel that releases amino acids slowly across roughly six to seven hours.
This is why whey is often called a fast protein and casein a slow one: the labels describe the shape of the aminoacidemia curve, not protein quality.
Why a sharp spike and a slow drip serve different goals
The fast whey spike drives a larger acute rise in muscle protein synthesis, which suits the period around training. The slow casein drip sustains a modest amino acid supply for longer, which is the rationale for casein before a long overnight fast.
In practice total daily protein and overall diet matter more than the timing of any single dose for most people. The diagram contrasts kinetics, not a claim that one protein is superior. General educational information, not a dietary prescription.