In the climb and descent, you are at a fixed power (Max Climb Thrust or Idle Thrust) so you pitch for airspeed. In level flight, you pitch for level flight and set the Target N1 for airspeed. Rather than make you guess the power setting, they give you one (and hope you can set it).
The FCOM states 'approximately 0.76/280' for severe turbulence penetration but then shows a table which states that these speeds are for the climb and cruise only. They advise turbulence N1 for the cruise.
Boeing gives you a target airspeed for all phases. The tables in
Performance Inflight have .76/280 listed for Climb, Cruise and Descent above 10,000'.
Boeing also adds in the
FCTM, "Turbulent air penetration speeds provide high/low margins in severe turbulent air." They further say, ". . . do not allow the airspeed to decrease and remain
below the turbulent air penetration speed because stall/buffet margin is reduced." Note they don't have the same warning here about being above. I'd guess that's why the target speed is closer to the top than the bottom of the limits.
So, for all who like to ride wherever the middle is, you're not following the
FCTM as you're "remaining below" the target speed.