question about resultant

Hello,

(%i1) resultant(a1 * a3 * x + b1 * y + c1, a2 * a3 * x + b2 * y + c2, x);
(%o1) - a3 ((a2 b1 - a1 b2) y - a1 c2 + a2 c1)

For which reason the gcd of the coefficients of x (a3 in this case) is not eliminated?

Thanks

Laurent.