He wanted to end it on his terms, that's all.' 'He ended it when Ireland were a hundred and sixty points ahead, the idiot!''He knew they were never going to catch up!' Harry shouted back over all the noise, also applauding loudly. 'What did he catch the Snitch for?' Ron bellowed, even as he jumped up and down, applauding withhis hands over his head. Most of the time, if the big point bump and game end caused by the Snitch being caught won't put your team over the top, you might try to keep the other player from catching the Snitch, in the hopes that your team will catch up enough, but if all hope is lost, you might catch it just so the difference won't be so much. It's likely that they looked at the spread, and at Krum's behavior over time in his games. Ireland may have had the better team, but Bulgaria had the better seeker. They were betting that Krum would see that, and end the game on his own terms. The odds were so far in Ireland's favor, that the point spread probably indicated that Ireland would be so far ahead that it would not matter who caught the snitch, and Ireland would still win.

If it was spotted by the Irish seeker, they were betting that Krum would beat him to it, because he's the best in the world. They were betting that Ireland would be too far ahead when it was spotted.

So the twins bet on a long-shot outcome, based on both those facts, and the idea of pride-that Krum would be too proud not to. That Krum would catch it, but not in enough time before Ireland had outpaced them. They knew that Ireland was the better team and that Krum was the better seeker.

Quidditch world cup goblet of fire