Definition from

Manhattan GMAT, page 15 of Number Properties strategy guide:

"A factor is a positive integer that divides evenly into an integer". So, at least in the GMAT world, consider only positive factor.

Factor and divisor means the same thing, and from Wikipedia:

"Divisors can be negative as well as positive, although sometimes the term is restricted to positive divisors. For example, there are six divisors of 4; they are 1, 2, 4, −1, −2, and −4, but only the positive ones (1, 2, and 4) would usually be mentioned."

So in the Math world you should consider also negative ones. But this is not the case.