impedance matching single vs series capacitor
The reason you see series capacitors at such low values is that the parasitic capacitance beomes significant.  With the multiple caps in series, much of the parasitic capacitance is also in series, so is divided by the number of caps.  The deliberate capacitance you get is also divided by the number of caps, but you can chose to make that higher to compensate.  

This trick works best when the individual capacitances are roughly equal.  1 and 3.3 pF in series does yield about 770 fF, but the result would be more reliable with two 1.5 pF caps in series.

Another reason two caps like this are sometimes used in RF circuits is for trimming.  The designer knows he wants a bit less than 1 pF, so puts a 1 pF cap in series with another part.  The resulting series value is then a reduced function of the second cap, giving you more resolution to hit the desired value from commonly available parts.  Quite possibly, the second value was found by experimentation for that particular board.  It may even be determined at production time from some calibration procedure.