This normally means that the shirt will be Y-.5. Thus, a 34/35 shirt will normally have a sleeve length of appx. 34.5. Sometimes, however, they could be 34 or sometimes 35. It is, in my experience, only cheaper manufacturers that use the double sleeve measurements. They probably do this because it is more economical -- they increase (theoretically) the number of consumers that could buy any one particular shirt size. Instead of guessing the relative proportion of 34s to 35s, they just lump the cohorts together and make one shirt for them both. IF they didn't do this and made a bad guess (say they thought that there was one 34 for every one 35; if the actual number was 1.5 34s for every one 35), they would both over and underproduce the correct shirts (i.e. they would overproduce the 35s and underproduce the 34s).
Thanks. I was assuming Y-.5, but it's good to be sure. Read the same thing somewhere, as well, that the cheaper manufacturers use X/Y.