In reply to dlong:
Dave,
You are correct that without the dist constraint, the number of solutions with 2 bits set (496) is much greater than the number of solutions with 1 bit set (32).
But these built-in iterative constraints are expanded out as an equation like
( int'(vec_1[31]) + int'(vec_1[30])+...+int'(vec_1[0]) ) dist {1:=90, 2:=10};}
so there is no implied variable solving order created by calling any of the built-in methods or functions.