When you call randomize() on an object of an Ext_A class, both dist constraints are in effect because you did not override the constraint in the BASA_A class. It will behave as if you had two dist constraints in the same class on the same variable. In that case it will only satisfy the range membership properties of the dist constraints, and weights will be ignored.
In your particular example, the dist constraint just happens to specify the same complete value range of a 4-bit random variable on both Var1_c and Var1_new_c dist constraints, and the dist constraint in BASE_A just happens to be a uniform distribution, which gives you the same results as if you had no constraints.