Hi:
From Office Online, nz "Returns the value of the specified field. If the field is blank, it returns zero (0)", whereas max "Returns the largest number in the argument".
Practically, that means that nz is only looking at a single field -- so if your field is, for example, in a repeating data group, it will only evaluate the first instance of the field in that group. Max, on the other hand, evaluates all instances of a field. So, if your field is in a repeating group, max looks at all instances of the field and returns the max. Does that make sense?