Imagine I hand you a sealed cardboard box and ask how long it will take to sort everything inside.
What’s in the box?
I’m not sure. Toys, maybe.
How many?
Somewhere between five and fifty.
How do you want them sorted? By color? Size? Type?
You’re the sorting expert. Use whatever best practices apply. I also have four more boxes, and I need all five sorted in the next five minutes. How long will it take?
Are the other boxes similar?
I think so.
You open the first box and see only a few items near the top. You estimate thirty seconds, double it for safety, and tell me five minutes should be enough.
At the bottom of the box, you find hundreds of marbles in different colors and sizes.
The estimate is now wrong. The stakeholders are disappointed. Everyone wants to know why a professional sorter cannot provide an accurate timeline.
Software estimates often work the same way. A precise answer does not make the contents of the box known.