The compounds released are highly volatile, meaning they quickly enter the air in their vicinity. Your eyes produce tears because your body wants to wash or dilute the chemicals before they effect your eyes. The tears are a natural way the body can wash the offensive chemicals away. Since there are a variety of onions, we would expect there to be a variety of amounts and types of sulfur compounds produced by them.
Onions are different in that way too.
One further bit of information. The chemicals which I described above are both volatile AND water soluble. This means if you cut the onion while holding it under a spray of water, the chemicals will wash down the drain and will not enter the air in as great an amount to cause your eyes irritation. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.