The 'S' is the number of cells wired in series - wiring in series causes voltage to add and capacity (mah) to remain constant, so that gives you 2 x 3.7v - 7.4v with 2650mah capacity.

The 'P' is the number of cells in parallel - wiring in parallel causes the capacity (mah) to add and voltage to remain the same, so your lipo now becomes a 7.4v 5300mah (2 x 2650mah) capacity lipo.

It actually has 4 cells as pointed out above and this scheme is usually done to increase capacity in the smallest space possible. When you balance charge this, charger will see 2 cells. This is how parallel charging works, all 4 cells will be balanced automatically by balancing the two pairs of paralleled cells.