Well, if you read the Engadget article that also references that chart, they did mention this doe snot also include iPads, and majority of the iPads are not mobile enabled, they are WiFi.
So there may be some serious Apple dedicated people who may be using iPad as their browser for locations where they don't need to use their iPhone and prefer the bigger screen.
What should be noted on that graph is also the fact that RIM's Web access seems to be dropping, no big surprise there, as I have found the RIM's web browser not to be the greatest, even on the same network, in comparison to the google's base web browser.
As for the decline in general for the iPhone web traffic, there are two possibilities.
1) The fact that the Android has appealed to a larger audience, especially due to the Verizon campaign since November. To be honest, the Verizon Droid campaign brought more attention to the Android OS than the several T-Mobile Google phone commercials, and I remember those as a "Why would I want to bother with google based phone?" when all they did was show celebrities showing off their phone with a picture on it. The Verizon Droid Commercial, on the other hand, went to the heart of piquing people's curiosity and did it well by putting it up against its closest competitor, which is the iPhone.
2) I believe the iPad and soon the better Tablet designed devices may be the next possible upswing for internet access material, mainly due to the fact that the push for touchscreens that are not 'huge' in comparison to the failed TabletPCs that were attempted with XP, The predecessor tablet devices were bulky and not super intuitive that in the long run, they were just really annoying laptops. The iPad and its following competitors will be leaner and much more intuitive that people will probably look at it as being a much better than using their iPhone or even Android based phones due to the larger screen and easier reading ability, especially for the vision impaired.
Although this might be an additional third reason - Diversity. In the last year, we went from just a few android phones to a literal plethora of them. Several in Sprint, some in T-mobile and a bunch in Verizon. With the iPhone mostly exclusive to the AT&T network, the decline in it may have been also fueled by the one thing Verizon has been harping on that AT&T is trying hardest to deflect, their service coverage. With more groups providing a smartphone that is on par with the iPhone, the interest in popularity shot up, especially with people who simply did not like AT&T's coverage of said areas.
I do know, that some iPhone users (at least, ones I've talked to) stated that if and when iPhone gets out of exclusivity with AT&T and is usable on other networks, they would switch... I would hazard to guess that people who wanted an iPhone, but hated the AT&T service would leap at the chance to go with iPhone and a surge would spike for iPhone for a period of time.
Quantcast's latest report reveals Android continues the upwards trend on it's quest to dominate the mobile industry. Up nearly 2% for last month, Android users now represent 25% of the total mobile web consumption. Apple's mobile iOS has been hit hard losing the most market share, down nearly 11% on the year. This past quarter we had a run of new high-end Android phones hit the US market including the Motorola Droid X, Motorola Droid 2, HTC EVO 4G, Samsung Vibrant, and Samsung Captivate. I'm sure that accounted for good portion of the growth. On the other hand, how much of the loss is reflected by the lack of inventory Apple experienced during the iPhone 4 launch?