Browser receive gzip response although IIS dynamic content module is not installed

Question

We have in production two sets of IIS 7.5 servers configured the same way: an ARR server handles the client request and routes it to one of two web servers depending on a specific user parameter.

All the IIS servers are configured exactly the same regarding compression: static compression is enabled, dynamic compression is enabled but not working because the "Dynamic Content Module" is not installed.

My problem is that although the compression settings are the same, one of the set of servers sends to the client gzipped content on a dynamic aspx page:

Request Headers
Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Cache-Control:max-age=0
Connection:keep-alive
Host:something.example.com
Referer:https://something.example.com/Ordering/OrderDetails/token
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.131 Safari/537.36

Response Headers
Cache-Control:public, no-store, max-age=0, s-maxage=0
Connection:Keep-Alive
Content-Encoding:gzip
Content-Type:text/html; charset=utf-8
Date:Tue, 13 May 2014 07:59:28 GMT
Expires:Tue, 13 May 2014 07:59:29 GMT
Last-Modified:Tue, 13 May 2014 07:59:29 GMT
Server:Microsoft-IIS/7.5
Transfer-Encoding:chunked
Vary:*
Vary:Accept-Encoding
X-AspNet-Version:4.0.30319
X-AspNetMvc-Version:3.0
X-Powered-By:ASP.NET

How can the client receive gzip content when IIS explicitly states that dynamic content compression is not configured?

Answer 1

Compression was coming from another component in our infrastructure: "Compression of HTTP reponses sent to network clients" was enabled in the ISA server configuration.