An unclear expectation is a future disagreement waiting for details.

An expectation nobody stated is a disagreement scheduled for later. Both people have a picture in their heads, both assume the picture is shared, and the difference surfaces only when the situation arrives and it is too late to negotiate calmly. The vagueness felt polite at the time. It was simply borrowing against a future conversation.

When an arrangement is being made, say the awkward specific out loud, the amount, the date, the division of who does what. It takes a minute and feels slightly uncomfortable. That discomfort is the whole cost of avoiding the later argument.