Rsbessa's observation is correct. This is common on the new B737NG's. After checking with senior pilots, the only reason for is to prevent the VNAV from going into a clacker.
Hence as seven3heaven mentioned VNAV reverts to VNAV SPD if
a limit speed will be exceeded, with COMMON VNAV and U10.6 or later
Although lot of pilots, are of the opinion, you can never touch the clacker with VNAV, irrespective of the FMC version.