I've attended quite a few conferences (virtually and physically) in the last year since Twitter introduced 'tailored trends'. At nearly every conference someone will make an announcement that the hashtag is 'trending worldwide'. Usually it isn't and in fact the person who has thought this is instead looking at tailored trends. These are based on who you follow and where you are; so that you see topics which are popular amongst the people you follow. It means that if you are at a conference with many of the people you follow, then it is quite likely that you will see that hashtag trending as a 'tailored trend'.
How do you know you are looking at a 'tailored trends'? If you just see 'trends' then this is 'tailored trends'. If a geographical area is specified eg 'UK trends' then you are not seeing tailored trends.
It can be a little bit more difficult that this to tell, so I made this screencast last week which I think explains why sometimes people are confused.
At the end of the day it doesn't really matter if your conference hashtag is trending across the nation or not! If it is trending amongst the people you follow that is probably a much more important metric.