I think you can find an answer here: [link=http://www.mat.uc.pt/~pedro/ncientificos/artigos/techbal.html]Grain and stuff[/link]. Just scroll this page all the way down...

It is description of the structure of the material. Depending how material was (is) cut you can get different material to bihave differently.
Sometimes material with different grain direction combined for strengths.

Good luck.