The Snapdragon 660 is used in several high-end phones while the 630 isnt getting much love. Both are mid-range (so affordable) 14nm chips (so power-efficient), but only one of them uses the faster Kryo cores. A new chip may be coming to bridge the gap between these two.
The Snapdragon 635 will also be built on a 14nm LPP process and will use an octa-core Kryo processor 4 big cores, 4 small. The rumor does not mention clockspeeds, though (the two clusters of the S660 run at up to 2.2GHz / 1.8GHz).
The GPU promises to be 20% faster than the Snapdragon 625 GPU. If true, thats not much Qualcomm was promising a 30% boost for the GPU inside the 630 over the 625.
A potential explanation is the Snapdragon 635 Plus the same chipset but with both processor and graphics running at higher clock speeds. Production for both is set to begin in the first quarter of next year.
Qualcomm allegedly also has plans for a Snapdragon 670 the first 10nm 6-series chip. It will also debut a new generation of Kryo core, the 360. Its not quite clear if the 635 chip will use a second or third generation Kryo. However, the reference to Cortex-A73 and A53 points to second gen (ARM already has the A75 and A55, which can serve as the basis for the third gen).
Source (in Chinese) | Via