Twitter is basically saying they establish a baseline for topics on the Social network. When a specific topics volume suddenly spikes they get included in the Trending Topics. Therefore sustained growth over a period of time (Ie. Tumblr and wikileaks) would not get included. I think this calls for a fact check but it seems like a reasonable explanation. I may spend a significant amount of time checking this data over the next week or month… but its interesting to say the least.