Isn't this Parkinson's law in action, i.e. work expands to fill the time available for its completion? That would mean you overestimate up to 2.4x each time.

Edit: math.

Oh, so if I estimate every task as 1 minute I'll finish really quickly?

The 2.4x is to overcome one's innate overoptimism (even if you see yourself as a pessimist), and to add contingency for unplanned complexity, bugs and general overhead.

It's a rule of thumb, not an immutable law, so if one has completed a similar task many times then less padding is needed.