Again, using the last generation as an example. The Gamecube and Xbox could do SO many more things graphically than the PS2 and DreamCast, and yet all four of them are one generation.
You're not getting it. The next gen moniker isn't in reference to how consoles released in the same period of time compare against each other, it refers to how that generation of consoles compares to the previous generation. Even though the Xbox was significantly more powerful than the PS2, they were both by every definition a significant leap over the previous generation, thus they were both considered next gen at the time and a part of the same gen as each other.