Text-search ranking is customisable[0] and results vary wildly based on the ranking behaviours selected and the weights assigned to different labels. It takes a bit of fine tuning and with the wrong parameters for your data set, you can definitely get results that seem unintuitive.

This should be a concern to the author, but there's no reason to think the search ranking is not working as documented.

[0] http://www.postgresql.org/docs/9.4/static/textsearch-control...