More than likely each saddle is a 1S2P 4200mAh. Then two 3.7V nominal packs in series to make the full saddle pack.

The cells inside are 2100mAh, two in each saddle.

In series, Voltage adds, but not capacity.
In parallel, capacity adds but not Voltage.

C never changes. Current capability goes up with parallel due to increased capacity, but C remains the same.