i need help to understand better this paragraph

151c Mistakes of Value If a mistake concerns the future market value or quality of the object of the contract, the mistake is one of value, and the contract normally is enforceable. Example 15.4 Sung buys a violin from Bev for $250. Although the violin is very old, neither party believes that it is valuable. Later, however, an antiques dealer informs the parties that the violin is rare and worth thousands of dollars. Here, both parties were mistaken, but the mistake is a mistake of value rather than a mistake of fact. Because mistakes of value do not warrant contract rescission, Bev cannot rescind the contract. The reason that mistakes ofvalue do not affect the enforceability of contracts is that value is variable. Depending on the time, place, and other circumstances, the same item may be worth considerably different amounts. When parties form a contract, their agreement establishes the value Sung buys a violin from Bev for $250. Although the violin is very old, neither party believes that it is valuable. Later, however, an antiques dealer informs the parties that the violin is rare and worth thousands of dollars. Here, both parties were mistaken, but the mistake is a mistake of value rather than a mistake of fact. Because mistakes of value do not warrant contract rescission, Bev cannot rescind the contract. The reason that mistakes of value do not affect the enforceability of contracts is that value is variable. Depending on the time, place, and other circumstances, the same item may be worth considerably different amounts. When parties form a contract, their agreement establishes the value of the object of their transactionifor the moment. Each party is considered to have assumed the risk that the value will change in the future or prove to be different from what he or she thought. Without this rule, almost any party who did not receive what she or he considered a fair bargain could argue mistake