One possible explanation for this is that as time multiplies with each dream level, the effects from the upper level would be felt gradually. For e.g. In level1, the van falls into the water
as a result, in level2, we get the zero gravity effect because the van is in free fall-
and in level3, we have the snow avalanche -
now, the van hit the bridge, started its free fall in water. Normally this would take about 20seconds. In level2 the free fall gives them zero gravity for a longer period, assume 10min(time is multiplied in inner dream levels). And in level3, this effect would occur even more gradually. So now we have 10min maginified, that makes it say 2-3hours. The other effects would have gradually occured in that level too. But as you know, the kick : level1 van hitting water, level2 lift propulsed with explosives resulted in the avalanche. That means, that before those effects from level1 could have reached level3, the kick from level2 actually reached level3. And the people got out of there quickly, they did not remain in level3 long enough for the effects to have taken place.