Here’s a try at phrasing it with less probability jargon:
The forecast contains a number of steps, all of which are assumed to take our best estimate of their most likely time. But in reality, unless we’re very lucky, some of those steps will be faster than predicted, and some will be slower. The ones that are faster can only be so much faster (because they can’t take no time at all). On the other hand, the ones that are slower can be much slower. So the net effect of this uncertainty probably adds up to a slowdown relative to the prediction.
Here’s a try at phrasing it with less probability jargon:
The forecast contains a number of steps, all of which are assumed to take our best estimate of their most likely time. But in reality, unless we’re very lucky, some of those steps will be faster than predicted, and some will be slower. The ones that are faster can only be so much faster (because they can’t take no time at all). On the other hand, the ones that are slower can be much slower. So the net effect of this uncertainty probably adds up to a slowdown relative to the prediction.
Does that seem like a fair summary?