Yes there are differences. For one thing some companies measure center to center while others measure outside to outside. So one company's 44 is another's 42. Also some bars curve in on the drops, so the top of the bar is wider than the spec. It always takes me a few rides to get used to a different width bar, but after a while your body adjusts and it's fine. There are limits of course, but really it's what you're used to.