The 920 isn't much of a flagship considering it's literally a year behind on hardware compared to the One and SGS4.
It's almost sad that Microsoft has once again shot themselves in the foot with hardware restrictions. In fact back when Windows Phone 8 launched several of us commented that dual core 720p WP8 devices would end up having to compete with quad core 1080p Android devices this year yet the MS fans insisted Windows Phone wouldn't fall behind on hardware again.
The hardware argument falls on its face because the iPhone 5 is Apple's current flagship, and will be for a few more months. I seriously doubt (thought it is possible) that Apple will implement a quad-core CPU in the iPhone 5S.
You're waging a spec-war across different platforms, and that simply doesn't work. Throwing more powerful hardware at phones isn't always of great benefit. Having a large amount of CPU power can help CPU benchmarks tremendously, but phones generally don't perform CPU-intensive tasks.
In Windows Phone 7.x/8 and Android 4.x, the GUI is rendered by the GPU. Web pages are rendered by the GPU in Windows Phone and in a lot of Android browsers. There are plenty of sites that, without GPU rendering, would choke; the fact it is used, however, makes them work perfectly well on either platform.
Games can certainly use large amounts of processing power, but the limiting factor often remains the GPU. There's nothing that current Windows Phones are remotely "slow" at, and the second generation (HTC Titan, Lumia 900, Samsung Focus S) have no trouble working just as well as the Galaxy S2 or iPhone 4S.
1080p support is supposedly coming in GDR3. Regardless, throwing more cores at phones doesn't inherently make them faster.
Also, the Lumia 920 was released around the time of the SGS3 and iPhone 5. The 925 and 928 are simply variants of the 920. When you consider this, the 920 certainly qualifies as a flagship. Nokia and Apple just happen to be behind Samsung's release schedule.