Why aren't .unity3d files being cached when viewed in the webplayer?

When downloading the unity model in a web page, I noticed that large files (for example 6.9MB) are not being cached even though the web server and browser are configured to do so. When I checked the request I noticed there was no "If-Modified-Since" header and therefore, no subsequent "304 Not Modified" response from the server. I did see however the .unity3d file stored in the browsers cache folder:

...\AppData\Local\Microsoft\Windows\INetCache\sample.unity3d

Why, if the file is stored in the cache, would the webplayer continually contact the server to download the file again?

Response Header:

 Response    HTTP/1.1 200 OK
 Content-Type    application/vnd.unity
 Last-Modified    Wed, 25 Mar 2015 13:34:35 GMT
 Accept-Ranges    bytes
 ETag    "713eff6e067d01:0"
 Server    Microsoft-IIS/8.0
 X-Powered-By    ASP.NET
 X-UA-Compatible    IE=EmulateIE7
 Date    Mon, 27 Apr 2015 15:50:42 GMT
 Content-Length    7237995

If I try the request using a simple jQuery ajax call with the appropriate request headers, the server responds correctly:

 $.ajax( { 
     url : '/path/sample.unity3d', 
     async : false, 
     type : 'get', 
     headers : { 'If-Modified-Since' : 'Wed, 25 Mar 2015 13:34:35 GMT' }
 } ).getAllResponseHeaders();

After the JavaScript is executed, I get this response from the server:

 Response    HTTP/1.1 304 Not Modified
 Accept-Ranges    bytes
 ETag    "713eff6e067d01:0"
 Server    Microsoft-IIS/8.0
 X-Powered-By    ASP.NET
 X-UA-Compatible    IE=EmulateIE7
 Date    Mon, 27 Apr 2015 15:57:58 GMT

Which is what I would expect to happen when the webplayer loads the file.