I'm not quite sure what your style/rhetoric distinction refers to. I conjecture that the underlying mechanism why we can often easily predict political content

I'm not quite sure what your style/rhetoric distinction refers to. I conjecture that the underlying mechanism why we can often easily predict political content from style is that most writers in general, and journalists in particular, don't write their texts from scratch, but mostly copy existing material and adapt it a bit. You can think of it along the lines of replication of DNA. The economics of modern journalism have made the copying worse. As an aside, I once developed software for an old telecoms protocol, and found a large number descriptions of this protocol in the trade press. They were all terrible, but felt quite similar. Eventually I managed to track down the origin: teaching material for a course developed by Cisco. As far as I can tell, not one article in the trade press was original. Yet, clearly some authors did quite a bit of work to disguise their copying, including -- bizarrely -- to redraw (badly) the figures in the original material.