Google's Android platform is gaining ground on its rivals in the U.S., such as Apple's iPhone and RIM's BlackBerry.

But Android did just NOT pass the iPhone in Web traffic in the U.S. last month, despite a story on TechCrunch with that assertion as the headline.

Based on recent comScore data, we estimate that iPhone-based web traffic is still at least 2X Android web traffic in the U.S.

So why did TechCrunch get it wrong?

The TechCrunch post is based on the latest monthly ad statistics from AdMob, a mobile ad network that's being acquired by Google. Specifically, one graph in AdMob's report showed that in March, Android's percentage of ad requests in AdMob's network surpassed the iPhone for the first time. (In the AdMob chart, Android ad requests are presented in red, and iPhone in dark blue.)

TechCrunch takes this to mean that Android Web traffic exceeded iPhone Web traffic. "AdMob measures mobile ad impressions, which is a proxy for overall traffic," TechCrunch's Leena Rao explains.

That's simply not accurate.

While Android's user base -- and Web traffic consumption -- is growing and could eventually rival Apple's, AdMob's ad request data is NOT a proxy for mobile web usage in general.

Why not? Because AdMob's statistics include an important variable that can cause significant fluctuations: AdMob's publisher relationships with web sites and apps, which cause different types of ad requests over time as the publisher network changes.

For example, if AdMob were to add more Android apps -- and therefore, more Android ad requests -- to its publisher base, or if it were to lose iPhone apps from its publisher base, that would affect its ad request statistics. In other words, Android and iPhone ad request stats could shift significantly from quarter to quarter with nothing else changing but AdMob's publisher relationships.

And to some extent, that's what's happening.

After speaking with AdMob, we understand that AdMob is seizing a big percentage of the new Android-app ad market. At the same time, it is likely losing share of the iPhone ad market, which is becoming much more competitive as other ad networks like Millennial, JumpTap, and Google catch up to AdMob's early lead. Even Apple is about to jump in the scene.

So while AdMob still has a strong position in the iPhone ad market, it's probably not RELATIVELY as dominant as it is in the Android ad market. Therefore, AdMob is probably capturing a much higher share of Android ad requests than iPhone ad requests, which is skewing the stats in Android's favor.

Putting it all together... iPhone users still outnumber Android users in the U.S. by at least 2 to 1, based on recent comScore data. iPhone and Android owners use their phones basically the same way, AdMob tells us. Therefore, overall iPhone ad requests should still significantly outnumber Android ad requests. Yet according to AdMob data, Android requests outnumber iPhone requests. This suggests that any discrepancy in AdMob's ad requests is a function of AdMob's ad network more than any other factor.

And therefore, it is not valid to say that Android passed iPhone in web usage using only AdMob's ad statistics as evidence.

Now, for something more entertaining: The Crazy Phones In Japan That The iPhone And Android Are Competing With