I'm pretty sure that's referring to the directory listing type, and not the operating system. Most FTP servers spit out a UNIX directory listing because that's what most clients expect.
Agreed, however when on my "Microsoft IIS/5.0 ASP.NET" server it returns back:
Host type (I): UNIX standard
My initial thought is the provider is using some form of Access Control List on a router or as said previously Layer 4 switching to redirect port 21(ftp) traffic to a Linux data server (the machine where the files sit). Then for HTTP traffic a Microsoft webserver is being used with a backbone (private) connection to the Linux box.