Increasing a single cell battery system over 10Ah is not trivial. First, the RK3399 and it's companion PMIC is designed with single cell applications primarily.

Increasing a single cell battery system over 10Ah is not trivial. First, the RK3399 and it's companion PMIC is designed with single cell applications primarily. Second, I don't know the exact reason why, but commonly available lithium cells top out at 10Ah. Even many fuel gauge ICs don't support cells larger than that. Adding another cell requires substantial development (design/source the pack, balancing and charging circuit) and extra BOM cost, whereas single cell involves copying reference design (which I know because I'm designing hardware product around RK3399 and ~70Wh battery pack) TL;DR; Can be done, but developers probably wanted to reduce the risk and stay on commonly walked path.