Jackie, your question is such a good one, that I've just spent a little time researching the answer. Here's my theory:

"Spiedino" is Italian for "skewer." Both spiedini and spiedies are cooked with the use of skewers. The differences don't seem all that big, really, just a slight difference of technique and serving presentation.
When I checked the translation of the words at an English to Italian website, 'spiedino' was listed but 'spiedie' was not. That leads me to believe that it's one of those words that has morphed from a legitimately foreign one into a slang English one due to repeated and common usage in a certain part of the country. 'Spiedini' just has never been called 'spiedie' here, that's all.