Where would you expect to see variations in handwriting

Handwritten letterforms vary between individuals. Every individual has different handwriting.
The reason for the varieties is the different writer's motor control program and the way this motor control program is executed. 
The handwriting is determined by the organic factors (physical anatomy and health, mental acuity, etc.) , but also environmental factors effecting the handwriting. These include the writing instrument itself, the writing surface and what lies beneath it, and other variables of the writing situation.