The critical power curve is a graph of tour maximum power output for a given interval of time. Each rider has a different combination of slopes and plateaus depending on their style and genetics.

The distinction between the riders above is simply a matter of different curves. Greipel/Cav have extremely high power over the first 30 seconds to a minute. Much farther out, their max output is substantially lower. Sagan's curve is probably a bit higher in the 3-10 minute range.

All of that is speculation ased on how these riders perform in varying circumstances.

As for training, I say train your weaknesses (raise your power curve) in your weakness areas. In the race, ride to your strengths.