The Trending Items model looks for items in your product catalog that have recently become popular (based on conversion events). This does not necessarily mean that the most converted items will be in the model.
The model is looking for an uptick in popularity of items. That means if an item was converted 10 times yesterday, but was only converted once before, 15 days ago, this is considered an upward trend in popularity and the model may include that item in the results.
The Trending Items and Trending Facet Values models require a minimum of 250 conversion events. Initially gathering data from the preceding 15 days and expanding to the last 30 days, if needed.