Its not complete nonsense. I posted that because I logged into my FedEx account and didnt see an option. I just took a quick glance. But, after reading a little further, Saturday delivery is only available with overnight and 2 day delivery through FedEx. UPS offers Saturday delivery with standard shipping. You just have to specifically mark it and it costs a little more. Thats probably why UPS was able to get a cheaper rate.
You're right, UPS to USPS is completely different, BUT someone mentioned it so I thought Id comment on it.