Note that every terrorist act is meant to cause actual deaths. If you want to see how "terrorism-prone" a group is, thwarted and failed attacks need to be included.
What the narrowed tell us is that Muslim terrorists are, on average, more competent and, consequently, more of a threat than other groups. This is actually the case, they're more organized than any other terrorist group since PIRA. They're also notably unconcerned with their own survival, which probably has an effect on their success rate.
BTW, in that data MP_Ryan posted, I was, quite surprised to see just how many attacks (most of them, in fact) are committed by separatists. Nearly all of them in Spain or France, which suggests that Basque nationalists are to blame. Why aren't those getting more publicity? It seems that two big, European countries have a major domestic terrorism problem. I didn't even know what ETA stood for (on indeed, remember that there is such a thing) until I checked the wiki.