when I compare day-to-day stats, my google analytics is sowing increased natural traffic, but my adsense Page impressions are reducing, what going on ?
The reason is that you are receiving untargeted traffic. Sometimes you will not find out what was going on. One of my sites went from 250 to 600 unique visitors per day, from one to another day. The sales even dropped ! One week later it was back to normal (alexa showed me as world top 20,000 during this time) and my sales even increased again. untargeted traffic is just a waste of bandwidth
If the adsense page impressions are going down while the page views in analytics is going up then you may be running out of ads for your niche. Another way this can happen is when a new page is hit that hasn't been "read" by google yet. Until adsense or search classifies the page it will often show public service ads. But this only happens for less than a day.
Also your visitors before might opening more of your pages and now, they might be come and leave immediately.
UV / Pageviews specially while monitoring Adsense PV might differ as a lot of people have JS disabled in their browsers or currently cutting out ads code while browsing the web In my opinion you should not depends on what adsense showing, after all you could refer to your website server logs.