BuzzBox has spelled out very well the logic used by the B777 FMC for the entry 'gates' to approach mode. During Descent and VNAV approach, the FMC VNAV functions well to maintain the profile accuretely, but makes no consideration for the Flap setting at the time.
During the pre-approach descent, speed is allowed to vary Up and Down to maintain the VNAV path accurately. Speed is allowed to vary +10/-20 knots from the programmed speed to maintain the descent profile. For example, a descent scheduled at 300 KIAS may be seen to operate between 280 to 310 KIAS (common). Not a problem at high speed, where even the lower speed limit is safe. The default speed below 10,000 feet is 240 KIAS, which, after allowing for the tolerable 10 knot increase, the nominal 250 KIAS speed limit below 10,000 feet is not exceeded. Perhaps, mach.865, this is where you have seen the 250 KIAS excursions.
The default speed during approach is 170 KIAS, which approximates the Flaps 5 manoeuvering speed. This is of course, like all other speeds, pilot programmable to a different value. The problem is that the VNAV logic does not consider the Flap setting at the time, and will, using auto-thrust, maintain the programmed speed irrespective of the Flap setting. Auto-thrust will intervene if the speed drops as low as VLS (well below manoeuvering speed).
It is not at all uncommon during training, or with a low experience pilot, to see low speed excursions resulting from leaving VNAV engaged for too long at the end of the descent and at the Initial Approach phase.
Now, if the pilot were to pre-programme all of the various speeds for the approach, 5 of them - Clean Speed, Flaps 1 speed, Flaps 5 speed, Flaps 20 speed, and finally Vapp with Flaps 30, and VNAV used, the pilot would have to take the next Flap setting when commanded by the VNAV re-scheduled speed, very messy indeed! It is infinitely preferable to pre-set the INITAIAL entry speed for passing the approach mode 'gate' (as defined by BuzzBox), taking manual speed intervention from there on, let VNAV do it's primary task of accurate profile management, and manually setting the new speeds as further Flap settings are made by the pilot.
It would be a rare day indeed if the speed / altitude / wind upon arrival was exactly as expected when the FMC set-up was made 200 miles previously.
If all of the fore-going made no sense, let me summarise it by saying that VNAV makes no consideration for the existing Flap setting or it's manoeuvre speeds, only VLS (last chance) is considered. Manual speed intervention is very very prudent indeed.
Regards,
Old Smokey