Why does the sentence “Mary has three fingers on her right hand” often imply that she has only three fingers?

In the philosophy of language, the sentence “Mary has three fingers on her right hand” often implies that she has exactly three fingers due to the Gricean maxim of quantity.

Proposed by the philosopher Paul Grice, the cooperative principle suggests that speakers should make their contribution as informative as is required for the current purposes of the exchange. If a speaker knows that Mary has four or five fingers but chooses to say she has three, they are failing to provide the most accurate, helpful information available.

Consequently, a listener assumes the speaker is following the cooperative principle and is providing the most specific information they possess. If the speaker meant "at least three," they would typically be expected to state the higher number to be fully informative. By stating "three," the speaker creates a conversational implicature that the quantity is limited to exactly three, as any more would have been mentioned to satisfy the requirement of being as informative as necessary.12