You see how the white box is k1/p1? You don't knit it and purl it both, you do them on different rows, RS or WS. The ssk is done on the knit row and when the st falls on the purl row, you p2togtbl. I don't know why she put the dec in for the RS/WS rows though as you don't do it on the WS at all. Maybe she used a charting program to generate the chart and just copied the key from it.