iis doesnt compress content only for requests containing "/gecko/" in user-agent

Question

Very weird behavior. My server returns gzipped response for all other requests.
in the images you could see that for user agent "/gecko/" the request is not gzipped and for "/gescko/" it is. i'm using the iis 7.5.
The content returned is dynamic, not static. here is the section from applicationHost.config. I'm deeply confused

<httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files" staticCompressionDisableCpuUsage="95" staticCompressionEnableCpuUsage="60" dynamicCompressionDisableCpuUsage="95" dynamicCompressionEnableCpuUsage="50">
<scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" staticCompressionLevel="9" />
<staticTypes>
  <add mimeType="text/*" enabled="true" />
  <add mimeType="message/*" enabled="true" />
  <add mimeType="application/x-javascript" enabled="true" />
  <add mimeType="application/atom+xml" enabled="true" />
  <add mimeType="application/xaml+xml" enabled="true" />
  <add mimeType="application/json" enabled="true" />
  <add mimeType="application/json; charset=utf-8" enabled="true" />
  <add mimeType="*/*" enabled="false" />
</staticTypes>
        <dynamicTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/x-javascript" enabled="true" />
            <add mimeType="*/*" enabled="false" />
        </dynamicTypes>