why this happens

Hello, 

Sometimes when Iam milling I home the machine, then I go to X0 Y0 Z0 position, I run the g-code and successfully finish the milling. But when I place the new same workpiece in same position as previously, then the Z0 position is in different location, in my experience was usually lower than previous. 

I think this happened also when I was milling, then I paused the machine, clicked again for milling from beginning and again the Z0 wasnt same.