from the Design & development: Death and dying:
120 hps for a 15th level fighter is strange because 7 x 14 is 98 and 120 - 98 is 22 even with CON 10 the fighter would get 12+con score + 7/level hp... 6 x 14 is 84, 120 - 84 = 36, that could mean a CON 16 fighter with 20 + con score + 6/level...