Well, nobody ever said that the area trend had to, or could be, or should be continued. When Galaxy's name was announced, I just thought of it as a nice revisit to that and nothing more. And, in Galaxy, Mario does not explore just one galaxy. Every LEVEL is a Galaxy, so you're effectively playing Super Mario Universe but with a better name. Super Mario World is named kind of weirdly, as well... but I suppose "world" has a different meaning in the realm of video games anyway, with every level in Super Mario 64 and every continent in SMB3 being called worlds. Or perhaps SMW was meant to be a huge twist in the way you play Mario games on the new Super NES: Super Mario Whirled! And some doof misspelled the title for the box and title screen.