Taking it simply, it means we are comparing Apples and Oranges. iPhone 6 has dual-core 1.3 GHz processor and Android devices have octa-core 2.1 GHz + speeds processors. For most of the part, android phones are used off the shelf SoCs. Even HTC is making a phone but they don’t have vertical integration but Apple has. The iPhone 6 Apple continues with dual-core processor which means iPhone 6 has just 2 cores.
The iOS powers the iPhone 6 which just needs dual-core processor but Android Lollipop and Android KitKat need quad core processors.

Now we can say that the Apple dual-core processor is the world’s fastest mobile processor. Yes, my dears! the A8 processor has just two cores with the speed of 1.4 GHz in the iPhone 6. It may be  remembered here that Qualcomm Snapdragon 805 is the fastest Smartphone processor available among android phones but Apple’s dual-core is faster than it even Snapdragon 805  has four cores with the speed of up to 2.7 GHz.
Now it is very clear that why Apple’s dual-core processor is faster than four or eight core processors. Apple always makes the right choice in designing its processor while Android is just interested in enhancing the count of processors.
It is remembered that Apple is making its own wider and more complex pipeline that focuses on maximum bandwidth. Such processors aren’t easily created. Only processors with longer and simpler pipeline are easier. Longer pipeline helps to show customers that their processor runs at 3GHz.
Companies aren’t created all the cores equally, so you shouldn’t buy processor by just looking its power quad-core or octa-core.
In simple words, iPhone dual-core processor is rapidly activated when you touch its screen even others android processors first reads memory, look services.
Now you’ve known the difference between iPhone Dual-Core and Android Octa-Core processors.