Hayanaratna 2.116

The reason is this. The first six parts of Virgo are Mercury's hadda (a planet's own small part of a sign). The last third is Mercury's drekkana (one-third part of a sign). The last three degrees and twenty minutes are Mercury's navamsha (one-ninth part of a sign). If the last navamsha of Virgo is taken, the drekkana and own-navamsha strengths come in full in its own house. But the maxim says: of one who wishes to join one, another slips away. So the high-point strength is a little less. The hadda and other strengths are also less, since the signs there are a friend's and the like. So the highest strength may be eighteen vishopakas, by one means or another, not twenty. For the others, the strength is surely less than twenty vishopakas, because house and high point differ.

Simple meaning: Why Mercury in Virgo cannot reach 20: its hadda, drekkana and navamsha parts do not all fall at 15 degrees, so the best is 18.