Morning Gooseberry,
The way the Trending function works is, every 5 minutes we get the list of top viewed threads from Google Analytics and we just display the first 5 of those.
So essentially, Trending represents the most viewed threads, not the most posted on (or the most recently posted on).
We are able to manually exclude things from the Trending block if we think they're attracting the wrong sort, though. 
Hope that explains things.