A thermocline is a distinct temperature barrier between a surface layer of warmer water and the colder, deeper water underneath. It can exist in both lakes and oceans. A thermocline can prevent dissolved oxygen from getting to the lower layer and vital nutrients from getting to the upper layer.
In many large or even medium-sized IT projects, there exists a thermocline of truth, a line drawn across the organizational chart that represents a barrier to accurate information regarding the project’s progress. Those below this level tend to know how well the project is actually going; those above it tend to have a more optimistic (if unrealistic) view.
This is all true, but the article assumes that everyone is acting rationally, in service to the stated goal(s) of the project, and that problems with the timeline are just honest mistakes. Unfortunately, in my 25 years, I’ve witnessed a nauseating amount of political infighting that sought to undermine projects in attempts to build and/or preserve personal power. This behavior employs the two things readily at-hand for ruining estimating: bad-faith technical decisions, and good, old-fashioned feet dragging. So the problem isn’t just people being wrong, there’s also a large component due to people actively sabotaging a project for their own purposes.
I’ve spent most of my career in Fortune 250’s, but I’ve seen this happen in a couple of very small companies too. As someone with a personality that is honest to a fault, this has caused me a significant amount of distress in my career. More than once, I’ve been the lone voice in the wilderness crying about the forthcoming train wrecks, only to be ignored, and then ultimately blamed for the crash, because I was the only one that people could point to for having said anything about it at all.