Good question, but I can't explain exactly why you don't always get a POST beep. My Z77 EX 4 board does the same thing, so your board is not abnormal.
As you said, it seems a POST beep occurs when starting the BIOS UI, or after changing a BIOS option, but only when certain ones are changed IMO. When I normally cold start my Z77 EX 4 PC, I don't get a POST beep. Different boards and BIOS' may do things differently, but IMO you have nothing to worry about.