Part of the explanation could be this:
When you are in VNAV PATH (not VNAV SPD), the A/Ps priority will, of course, then be to follow the PATH. If you for some reason end up above this path, the FMC commands a low speed, e.g. 210 or 235 in order to ensure that the thrust levers remain at idle. Why? The FMC "knows" that you must fly at a higher speed in order to follow/ catch up with your path. Since it is not desirable to use more thrust to achieve this higher speed, the FMC commands a LOWER speed to prevent the A/T from adding thrust. Given that you are in PATH mode, the A/P will then naturally have to dive for the path. As you catch up with your path, the FMC will gradually command a higher speed until once again at you pre-selected descend speed. You could go to the descend page and select SPEED i.s.o. PATH if you really wanna follow a (ATC?) given spd.
Does that explain it?
