This is a feature I have observed on most descents. You instinctively know when the descent information supplied is way off beam. Suddenly it just seems to go haywire and you get data that must be anything up to 7000' out. It is mystifying. Previously I flew the 747-400 with the same flight system. The descent data was consistently accurate and reliable, with hardly any incidences of faulty descent data. The 737NG descent information is largely a joke. So inaccurate at times that you can never depend on it at all.