This was a mass produced "every man's" bike, and not one of Yamaha's flag ship models, so I'm not surprised. They probably didn't pay much attention to stuff like that. I'm not sure it matters all that much though. All this part does is rock back and forth. If you were dealing with 4 spinning parts then, yes, weighing the same would probably matter.
I've never had the ability to weigh small items like this accurately but will soon when my Ohaus triple beam gets here (thanks again eBay). I plan on checking lots of this stuff. The rockers seem rather crudely cast with stiffening ribs that vary in size quite a bit between different ones. I'm sure you could carefully grind some of that away to even things up.