Glad to hear you're enjoying the thread. I know I don't make many, but I try to answer as many questions as possible when I do.
To be perfectly honest with you, I ask myself the same question almost every day I go into work. I can shed a little light onto the subject for you though.
One issue is regional sales. It's the same reason you can ask for a soda some places and they have it listed as pop. Or the same reason you can't ask for bull testicles in a store most places- you have to ask for mountain oysters or Rocky Mountain oysters. Beats the hell out of me why we can't just say what it is and call it a day...
The big ones for me when I first started my newer job were that we used the same naming methodology for pork cuts that we did for beef. Pork new york strips, pork top loin chops... things like that.
In addition to regional sales, naming gets changed because certain parts of the muscles are removed for one cut, so the cut of a same name looks different. Baseball sirloins are never sold in my area, nor are Denver steaks, and even rarely (unfortunately) tri-tip steaks. We use the muscles differently, so the name of the cut is different as well.
We don't sell just regular sirloins at work, instead we have to call them full-cut, fillet, strip, and other names... It's confusing, but it gives an EXACT name to the muscles you're getting for dinner.
The problem that you're probably thinking of is when you go into a store for that sirloin steak and it says "top sirloin butt" or "full cut boneless sirloin" or "Steakhouse sirloin"- while the cut is either completely the same, or it is only slightly varied. I really don't get it either man.
The USDA does have names for everything, but due to old school cutting and "new" cuts being developed over the years, some of those steaks, roasts, and even sub primals can have completely different naming across the board.
Unfortunately, I feel it would just be too hard to get the same labeling across the US.