This tendency is known as the planning fallacy. We picture the focused version of the task: write the email, finish the deck, repair the shelf. The actual day includes setup, decisions, interruptions, missing information, and ordinary variation.
Past experience can help, but only if we let it. One classic study found that people often remained optimistic even when their own earlier tasks had run late. Looking directly at comparable past work produced better estimates.
Steady adds friction when a plan exceeds the capacity you chose. You may still add the work. The point is to make the trade-off conscious, not to let optimism silently set the size of the day.
A realistic plan includes uncertainty. It is not a promise that nothing unexpected will happen.