Neat. I think in addition to strength of the metal, the design of the spoke plays a part, As you mentioned, the spoke heads had different designs and might have different results in real world use. I experianced this when building up a tire with spokes that had a rather large bend at the spoke head. I was getting breaks at the heads and, when researching the issue, I found This which described my problem pretty accurately. I did what Peter did and got some Wheelsmith spokes to relace my rim. Haven't had a problem since.
But, as you pointed out, actual strength may not vary by that much. While the design of the sproke may have an effect, the biggest effect is probably even tensioning and truing. Relacing your rim is a reasonable precaution on a touring bike (especially when spokes have already been breaking) but it's possible that a retensioning would have done the job. Once spokes start breaking, it's hard to know if the remaining spokes have been weakened or not, but I've heard that retensioning before a spoke breaks can go a long way towards fixing a poorly built wheel before it becomes a problem.

While I didn't want to cut corners on my 700c, touring bike wheel, I did make a 20 inch wheel for another bike using spokes salvaged from junked bikes at the local bike co-op. That wheel went together fine and has never broken a spoke. Of course a 20 inch wheel is a bit sturdier to start with, plus its a front wheel, so it's never seen the stress that a rear, large, touring wheel has, but it's nice to see that those old spokes still have some life in them.