would have been 一定为虚拟语气?

题目

would have been 一定为虚拟语气?
The London Olympics are about to begin,and spectators will again be riveted by feats that would have been impossible when the modern Olympics began in 1896.
为什么该句用了would have been impossible?“本应impossible的,但事实上不是impossible”?然而,从内容方面讲,在1896年本就impossible,事实的确impossible,并无虚拟啊.

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