Why do you assume that this has anything to do with the percentage that Google gives to the publisher? That could be the case, but there are other possibilities that are just as likely. For example:
1) An increasing number of advertisers in your topic area may be turning off the "content ads" option in AdWords after learning that their ads are being served on third-party Web sites and not just on SERPs.
2) As the pool of publishers expands, higher-bidding advertisers may be exhausting their budgets faster, so that more of your readers' clicks are on lower-paying ads.
3) Depending on your topic, seasonal factors may determine supply and demand for keywords (which in turn will affect bids).
As far as the Google-publisher split is concerned, it's possible that Google has cut the publisher payout, but it's also possible that payout formula has been altered to reward higher-volume or more profitable publishers. In other words, the average payout may not have changed, but specific publishers' percentages may have gone up or down. (This isn't even conjecture on my part; I'm simply talking about possibilities.)
Ultimately, your bottom line is what counts--or, if you're comparing revenue sources, your effective CPM.