What is the surprising game change that is announced? Why is it so odd that new rule has been added?

The surprising game change that was announced was that from that particular year, two tributes could be won as long as the two winners did not come from different districts. It was odd that a new rule had been added because there initially seemed no need of adding it.

Explanation:

In the eighteenth chapter of the novel named 'The Hunger Games', Claudius Templesmith (the announcer of the games) announces a new rule of the game that surprises everyone. This rule is about the number of possible winners of the game which has now been raised to two from the earlier one. The people are surprised after the announcement as they are in the state of baffle thinking how can there be two winners of a single game.