[aloiz]

2 is the number of ways A2s and AKs can make a str-flush
2*2 is the number of ways 23s and KQs can make str-flush
etc..

It would have been clearer if I had written it out like:

2/13 * 1/C(50,3) + 2/13 * 2/C(50,3) + 2/13 * 3/C(50,3) + 7/13 * 4/C(50,3)

2/13 times your suited connector is AK or A2 in which case you can only make your straight flush one way. 2/13 you have 23 or KQ and can make the straight flush 2 ways, and so on.

aloiz