I am pretty sure they measure from center-to-center at the ends of the bar. I just bought some nitto bars that the seller gave a 42cm measurement outside to outside (at the ends), and when I got them they were marked 400 (40cm) and indeed were too small for me. I have checked some other bars and found the same.
I also bought some 44cm noodles a while back and found out immediately I did not like them at all. I'm 5'9" with about 18" shoulders. Actually hurt my elbows (I don't know why) to ride them with my arms spread out like that and I didn't like the bends near as much as the bars I prefer. But then, I think most people try different bars and eventually find one or two they love.