Given that the most ready 64-bit IP from ARM is the Cortex A53 (successor to the Cortex A5/A7 line), and all of the sudden it makes sense why Qualcomm's first 64-bit mobile SoC is aimed at the mainstream market (Snapdragon 400 instead of 600/800).

ARM included a number of power efficiency improvements and is targeting 130mW single-core power consumption at 28nm HPM (running SPECint 2000). It's expected to see slightly higher power consumption at 28nm LP on this extremely low power design.
- Anandtech