Interesting find. I've dealt with the cast and stamped steel impellers on the 3100 pumps. I personally prefer the cast impeller, but the stamped steel has worked fine for me before (could be some other design factors).
Thanks for sharing what you found, I was on the band wagon thinking air pocket.

As an FYI, I read that the 3800 Series II from the factory has a stamped steel impeller and you are best off going with a 3800 Series I pump/pulley or a 3800 Series III pump, as both fit up fine and have a cast impeller. The change in flow is not much, but if every little bit counts, that's a worth while switch (and not expensive). If my pump ever gives up, that's on my shopping list.