We've seen recently from some of these systems that the POWER processors are much more effective than x86 processors. They consume lower power and generate less heat.
In other words, POWER is not in the same market as x86 or ARM. It does have its own niche in the market.
Look at that Top500 list again and compare #1 (x86) and #3 (POWER). #1 has about double the number of cores, performance and power usage. I'd say the high-end Xeons are comparable to POWER in all aspects.