I guess everyones experience is different. I am normally using gas shielded electrodes, but I the data in the catalog almost always exceeds what we get. The electrode manufacturers have a specific way they are required to test (i.e. one or two passes per layer for layer 1, 2 or three passes per layer for layer two and up, 10-17 total passes for .052 through 5/64 wire, etc.) so the electrode perfoms well under these welding circumstanses, but is not guaranteed to perform as well under others. Some of the filler metal companies also have their technique down so that the two layer passes overlap in the middle and their charpys deliberatly include the maximum amount of grain refined weld metal. But once you get it out of the lab, and the test conditions are not very similar to those in the AWS specs, the CVNs (in my experience) drop off.

The addition of nickel is especially beneficial to self shielded electrodes since there are two main ways to improve charpy results. One way is to get really clean metal, and the other way is to add beneficial alloying elements. Since self shielded FCAW by the nature of the process cannot produce very clean weld metal (lots of oxide and nitride inclusions), the easiest way to do it is with the beneficial alloying elements.