I've read more direct info on the subject than what I linked to below, but basically the theory is that when a person want to claim credit, they're much more likely to use words like "I", whereas someone who wants to deflect responsibility is much more likely to use a pronoun like "we", or third party pronouns.
I heard a story on NPR where a professor listed to recordings of CFO's quarterly investor calls. When the CFO used "I" when discussing his/her views of the company's performance, the performance tended to be positive; when he/she used "we" or "the company thinks", the performance tended to be negative.
Here's a broader discussion from a Harvard Business School article:
http://hbswk.hbs.edu/item/7234.html
How to Spot a Liar
- Liars used far more third-person pronouns than truth tellers or omitters. "This is a way of distancing themselves from and avoiding ownership of the lie," Van Swol explains.