The producer surplus is objective because the it is the total revenue minus the total variable cost. For example, if the producer sold 100 goods at 5 each, and the total variable cost was 300, then his surplus is 200. The total revenue and variable cost is object because it is numerical, a seller can not change these numbers based on his own perceptions or feeling.
But the demand curve is different from consumer to consumer, the consumer surplus is defined as The difference between the value that a consumer places on units purchased and the amount of money that was required to pay for them. The consumer places on the good is entirely subjective, therefore is it correct to say how big your consumer surplus is entirely up to the individual consumer? And even for the same consumer, the consumer's value placed on a good can change from time to time, for example, the value I place on going to see a movie on a date with a possibility of getting laid later that night is much higher than the value I place on going to a movie by myself even thought the product is exactly the same. But my two consumer surplus would have a enormous difference.