Why there is so much variation in skin permeation study (in vitro)?

I completed in vitro skin permeation study using franz diffusion cell with human skin. Every time formulation is same and skin is also from same donor. All the procedure is same but why there is more variation between permeation (ug/cm2) (n=3)? Can some one tell me possible error and steps to rectify these error?
The experiments even if done on two different days should not have much variation. SD and SE is coming so high (30-70 ug/cm2). My sampling method is same every time. I do not face this problem with release study using dialysis membrane using same sampling procedure, medium and HPLC method then why I am facing this in Skin permeation using franz diffusion cell? Is this normal (give reference) or tell me possible cause of error to rectify this.?