That summation index variable n1 is not entirely like the variable n. That name n1 represents a bound variable, since your sum call denotes that it would only take integer values between 1 and n-1.
There's not much significant difference between using the name n1 instead of the name i, or what have you. For example, these next two are conceptually similar:
sum(f(i), i = 1 .. n-1)
sum(f(n1), n1 = 1 .. n-1)
The end result of both of those would usually be the same; the results depend on (any value of) n in the upper end-point of the index, but the results don't depend on which choice of name is used for the summation index.
Maple knows a bit of that, which can be useful:
expr := sum(f(i),i=1..n-1):