I believe there is an oversight on the creation of this Table: ATCGTGGTACTG - Index A TGGCCCTGTATC - Index F Both have length a of 12 they both have only 1 common letter between them (the second letter from the right) so 12-1=11 not the 10 printed in the Table itself.
Isn't this technically the WPGMA method and not UPGMA? UPGMA takes into account the number of clusters when combining the rows whereas this just takes the average of two.