Can you give evidence of this? Because i'm really highly doubting there's any truth in this. I don't think it would be very kind to mislead people into thinking they shouldn't care/double check their solutions cause they would think they would get full marks for putting some random solution on the paper just for the fun of it.
Also why would they do that if they were say trying to differentiate the state rankers and the ones who just missed out on it. I'm sure there are many people who would get 100% if not for calculation errors.
I can vouch for what braintic claimed.
HSC marking is much more flexible than what most students think. Students are usually deducted marks for demonstrating a non understanding or misunderstanding of Mathematics on their paper. A calculation error is not demonstrating either of those.
Of course students should be careful when making their calculations. But in the case that the student is clearly demonstrating that they know what they're doing, but just miscalculated, they may still get the mark.
Also, a lesser known fact is that the marking criteria is relaxed further as you go deeper into the paper. When you hit say Question 16, then the markers will be finding excuses to give you the marks! If the question requires at some point the double angle formula in the working out and you just randomly write down the double angle formula somewhere on the page, chances are you'll get a mark for it.
So conversely, if it seems that many students are getting full marks for a question and the mean is 14/15, they will try to lower it by tightening the criteria slightly.