If you look under the adverse weather section in the Supplementary Procedures of FCOM1
PHASE OF FLIGHT AIRSPEED
CLIMB: 280 knots or .76 Mach
CRUISE :Use FMC recommended thrust settings. If the
FMC is inoperative, refer to the Unreliable
Airspeed page in the Performance–Inflight
section of the QRH for approximate N1 settings
that maintain near optimum penetration
airspeed.
DESCENT: .76 Mach/280/250 knots. If severe turbulence is
encountered at altitudes below 15,000 feet and
the airplane gross weight is less than the
maximum landing weight, the airplane may be
slowed to 250 knots in the clean configuration.
I know the AFM supersedes the FCOM but why the difference then?