A little puzzled about length...

   I am at a length of 15 to 16 inches or so all over my
head and yet on me on the front and sides i am
armpit length, and in back im about an inch away
from brastap,, i have seen some ppl say those
same inches 15 or 16 are shoulder length on them
lol,, and i just read someone say that they are 21
inches and thats armpit length for them i dont have
an unusually short neck or small head but i do have
a small face ,, could this be why my hair hangs
longer lol,, im puzzled lol or am i measuring
something wrong?? what i did was take a tape
measure and sort of hang it from where i have my
hair parted on the sides and just see where it
reaches,, is this flawed in some way??