I've always played that the 20 counts double. This may be the second time you were wrong.
Now that that's out of the way....
That's the problem you get when a game has multiple rule sets. There have probably been 10 (or 20
) different versions of 10 ball rules. I would go with the WPA rules for 10 ball even though I don't particularly like them. The best alternative is to ask a regular who was not watching the game how he usually plays or how he has seen people play and be careful not to let him know which player made the 10.
And in case anyone was incredulous or wondering what a 20 ball looks like, here you go: