people have bigger or smaller heads, longer or shorter arms, necks, legs, torsos, etc. the whole 'seven and a half heads tall' thing is just a general 'average' taught to noob art students who would otherwise draw people that are hideously malformed and disproportioned.

there is not one set of mathematical proportions that defined the entire human race.
many people are built differently, and therefore a model can be perfectly anatomically correct yet not adhere to the 'seven and a half heads tall' guideline or any other art 101 guideline. these are just guidelines and teaching aids, not firm rules.