You are right. I found the following lines in Orchard.Web\Modules\Orchard.OutputCache\Filters\OutputCacheFilter.cs
// create a unique cache per browser, in case a Theme is rendered differently (e.g., mobile)
// c.f. http://msdn.microsoft.com/en-us/library/aa478965.aspx
// c.f. http://stackoverflow.com/questions/6007287/outputcache-varybyheader-user-agent-or-varybycustom-browser
// enabling this would create an entry for each different browser sub-version
// response.Cache.VaryByHeaders.UserAgent = true;
I think that this way can affect caching performance because in most cases we have same responsive theme for all devices. At least we must create an option on setting page?
What is your opinion?