Cache frequentHitThreshold problem

I configure my website with user-mode caching:

<location path="images">
<system.webServer>
    <caching >
      <profiles >
        <add duration="00:10:00" policy="CacheForTimePeriod" extension="*" varyByQueryString="*" />
      </profiles>
    </caching>
    <serverRuntime enabled="true" frequentHitThreshold="2" frequentHitTimePeriod="00:30:00"  />
</system.webServer>
</location>

Next I make some requests:

1. http://127.0.0.1:81/images/123/?hight=100&width=100 - (this url does`t out to the cache, because the first request)
2. http://127.0.0.1:81/images/123/?hight=200&width=200 - (this url put in cache, but it different from first url and frequentHitThreshold=2)

Why second url was immediately cached?
Does frequentHitThreshold observe url without querystring?