At times you will see that the Google Adsense only shows 2 or 3 links in n a 336 x 280 or 300 x 250 pixels slot on specific websites or pages where it could easily show 4 links. But if you go to the bottom of the ad slot and click there, you can see there are actually 4 links on the same slot. Well this may tense you up for losing CTR due less dense ads then you should read on.

Google automatically optimizes the expansion of ads to provide you with more eCPM. That implies, if someone will click on any of the 3 links in the ad slot, you will receive more returns as the marketers are willing to pay more for keywords on your website. This is known as Expanded Text Ads As per the Google support page about ETA,
When you see that the text ad has expanded to occupy more space or complete ad unit, you are either looking at the keyword targeted text ad or placement targeted text ad

As per Google “Placement-targeted text ads appear when advertisers select specific sites on which to run their ads, on a pay-per-impression basis. Their ads are then expanded to fill an entire ad unit, making placement-targeted ads even more appealing for advertisers, and resulting in higher bids, meaning higher earnings for our publishers.
Keyword-targeted text ads may also be expanded when we determine that a larger ad will perform better for a particular page. Our AdSense technology will automatically determine the optimal number of ads to display and will only show fewer ads when it improves monetization.”

Please note that at this time, it isn’t possible to opt out of displaying expanded text ads. The number of ads shown in your ad units is automatically determined in order to maximize clickthroughs and revenue.

As per Google’s note you will receive more returns if Adsense is showing fewer links in the ad slot whether it is 300×350 or 336×280 pixels.