Our SDK logs mediation requests just before it loads the mediation adapter for the target network. The usual sequence looks like this:
1) AdMob is asked for an ad by the SDK
2) Admob logs that iAd is the next mediated provider
3) SDK loads iAd Adapter
4) iAd Adapter request an ad
5) iAd server logs that a request is made
A discrepancy between iAd's number of requests and AdMob's number almost always indicates a failure in step #3 or step #4. I would recommend verifying that you're working with the latest version of Google Play Services, our SDK, and the iAd Adapter & Framework. Once that's done, if the problem persists, check the logs for anything that indicates the AdMob SDK is having trouble loading or using the iAd adapter.
-Andrew