This is because at this stage of processing, the trees built bottom up for 'which fruit', 'John' and 'have' includes two NPs with LHS category N P S (Figure 6) while the r8 rule only requires one such NP.
adjunction is handled as an optional requirement which can be satisfied by the adjunction of an empty string and subscripts indicates the nature of the requirement (S for a substitution and a for adjunction).