Oso
Premium Pilot
Probably because it is not uniform. Some get only an unzipped ipsw file (like me for example) and some get a zipped file (like you for example). It just depends on your OS and your systems settings.I'm very surprised that all those websites that offer the downgrade iOS 8.4.1 don't inform or remind the downloader to change the .zip/rar extension to the .IPSW extension?!
Since it is relatively common to get either one or the other, I totally agree with you that it would make more sense to note that on all of those sites. I guess they must think that a large majority of ipsw file users are computer / gadget experts who can just figure stuff out quickly.
Cheers!