Why does my carefully planned week fall apart by Tuesday?

Every Sunday I make what looks like a realistic plan. By Tuesday I’m moving half of it forward, even when I’ve been working all day. The usual advice is to use a better planner or be more disciplined, but I suspect I’m pretending every open hour is usable and estimating everything at best-case speed.

How would you test that without building another complicated productivity system? I need a small experiment that tells me whether the problem is overload, interruptions, bad estimates, or work that depends on other people.