As I read it (from our FCTM and other documentation) Boeing's logic behind not wanting ace pilots setting their committed to memory pitch and power figures for, e.g. for 250 knots 'cos they think they are at 250 knots and S&L is because by the time they recognise the situation they might not be at 250 knots and they might not be in level flight....so that's why they came up with the "it's safe" figures
Wiggy is correct plus they also state "at any and all weight/altitude combinations, the aircraft will accelerate from low speeds, and slow from a high speeds.... At light weight and low altitude, the true airspeed will be higher than normal, but within the flight envelope ".