Horses are different and they are certainly different in their sensitivities in the mouth, and consequently, in the way they respond to the bit. You may see a horse later in his career and see that he is wonderfully soft in the bridle, seemingly ridden on “reins of silk,” and you think what a wonderful job of training someone did. Another horse requires a much more “positive feel,” and you might think of this as a proof of less skillful training. You have to be careful because this perfunctory assumption might not take into account the horses’ natural sensitivities, which they were born with.
This is not an excuse for heavy-handed riding. In the end, it is not lightness in the bridle that is the real test of training, it is lightness in the forehand—the ability to develop the strength and elasticity of a horse so that the rider feels the horse can elevate the forehand whenever the rider wants. Some horses may take more or less contact by nature, yes, but a controlled shift of balance to bring the forehand up to a levade or to accomplish a well engaged halt from an expressive trot just from the aids of the rider’s seat and back requires the correct training.