Humans are one of the smallest common races, standing at a mere 6 foot (size 4).
(Creativity, Strength and Vitality fulfilling their respective letters.)
Get ready for first paragraphical hypothesis...
Creativity - Free will, action points, and a lot of super stuff.
Reflexes - Saves and initiative and some abilities.
Aptitude - Skill points.
Vitality - Constitution and endurance.
Influence - Charisma and the 'luck' stat; it affects all others slightly.
Nimbleness - Actions/turn and avoiding danger you are aware of.
Grit - Fortitude and will; both saves or tests or required for use of some abilities.
Strength - Lifting, speed, and base melee.
No ability can go higher than 10 max or 1 minimum, if you're human - unless you have some external benefit/negative such as being on drugs or physically/mentally disabled/insane. Also, the stats are meant to define the core character - each stat directly constributes to an actual thing the character does in some way, whether it's casting a spell, or performing a more specific ability or combat technique, etc., etc. In this way, the set of skills a character picks could be his 'class'.
EDIT: Sorry if that's too non-open for interpretation. I got a little ahead is all. -_-