<a href="http://www.flickr.com/photos/28494814@N07/2660333328/" title="pair by X101, on Flickr"><img src="http://farm4.static.flickr.com/3014/2660333328_8a98c966e1_o.jpg" width="520" height="280" alt="pair" /></a> So today traders apparently felt that EXXON was worth almost 4% more than chevron as compared to just yesterday and the last 4 days for that matter. We're talking about billions of dollars.
I just pulled up my charts to answer your question. On my primary chart, where I use an uncommon interval. The RSI on XOM hit the lower band (I use 30). CVX unfortunately has still not reached that territory. That would be my explaination for this. It is quite a common thing. Guess this is why the term non-systematic risk becomes relevant.