I was under the impression that was the correct size. Tires come in gradations of every two mm or so, and tubes are in larger gradations. You get the tube with the range that suits the specific tire width.

(kinda like jeans come in sizes 2,4,6,8,10,12,14,16,18, etc and underwear comes in S,M,L,XL)

I love my LBS, cuz they are willing to explain everything to me, even when all I'm doing is buying a tube!

www.recycledcycles.com