It's crazy but it's how it works.
For example, if you agreed to sell me a country house for £500,000 then changed your mind. I could sue you for the difference between the current price and our agreed price. But if the property market crashed between now and the time of judgment, my claim will become worthless.
Conversely, if a day before the hearing, we discover that the house has a secret tunnel leading to hidden Nazi gold, why shouldn't I should be entitled to the value of that?? Of course if you were smart you'd immediately come to me once you figured the true value of the property and reach a settlement before the true state of affairs is revealed, which is what we should have done...