The black&white stamps shows the 2 blocks available in one half-pane. That leaves 11 "orphans" that cannot complete a set of 7. You wind up with one 508a, and 2 each of b,c,d,e, and g.
If I were to venture a guess, I would say that Scott is being lazy and simply using a generic (so to speak) valuation formula. With no 508f available to match up with one of the 11 orphans, it is the key to this set, so it gets the $13 valuation, then they just slapped a $2 figure on the rest. The b,c,d,e,and g stamps probably merit somewhat less than a $2 value, but 508a should be higher.
I just checked Michel 2010 for Australia. They confirm my feelings. They put a 1.50 Euro valuation on the 5 common stamps. Scott values are typically 50 to 70% of Michel values, and 1.50 Euros is only about $1.90. Michel values the 2 short-printed stamps at 10 and 11 Euros (about $13 and $14.25).