A quick look at an online vendor shows that the vast majority of mt. bike bars are 31.8. Same with road. Some are 35 which is silly, there are a few 25.4, but most are 31.8. Seems the mount manufacturers needed to find a standard and settled on 31.8 as its easy enough to shim to a narrower bar, many provide the shims as well. Theres absolutely nothing wrong with putting a shim under the mount, its often needed for lights as well, not getting why this is an issue for you.