[Blarg]

Well, I just find manufacturers differ a lot and sizes are only a rough guide at best.

If you have a regularly proportioned body, you probably have a wide safety margin. But if you are especially wide or narrow or tall or short, or long-waisted or short-waisted, you've got to really be there in person or the differences between manufacturers sizes can make a big difference to you.

Me, for instance, my arms are long. If I just go for a simple size like large or extra large(I can often wear either, too), sometimes the sleeves are too short and sometimes they're not. If I buy a dress shirt, where the sleeve length is measured, it can help me zero in on the size, but still there will be variability.

Maybe some guys aren't sure if their bodies have the average proportions, but I know my arms are just a bit long, and it's enough to make me extra careful when buying stuff.