I think previous poster is correct! Flew 737's a few years ago and went through several FMS update cycles. Started out with a descent logic which was based on a level and dive profile to a continues descent profile.
Meaning: the old software, if you had a descent profile which was below an idle descent profile the FMS would program your descent so as to start descent towards the first constraint with an idle descent, then VS 1000fpm until the next idle descent.
A software upgrade of the FMS came and the logic was changed. The FMS would start an idle descent to the first alt constraint and from there on a geometric descent to any subsequent alt constraints. However, the FMS logic did not correctly take into account decelerating requirements!! Leading to some weird speed indications. Later software updates went to the old level and dive logic.
They way around it was, if memory serves me right, just before you hit an altitude constraint, delete the altitude constraint (before capture! You may miss an altitude constraint by 2 or 3 hundred feet) you will force the FMS into the 1000FPM until the next idle path descent and the idle path descent will take into account the decelerating part.
Hope this make sense and helps!