Okie dokie, I'll bite.
From the first post, if you do not allow your isolated compounds to completely dry at the end then when you weigh them your weight will be high because you will be weighing the water along with your compound. I forsee that this will give you higher weights than what you actually have.
I did not see silver nitrate mentioned in the cards or the PDF that you linked to. It is commonly used in older chem analysis methods as a way of measuring the amount of chloride in solution as it precipitates out of water as silver chloride, but I doubt that it is being used that way here. I see that it is being used to "test" for the presence of salt in the water filtrate. In doing so it will use up some of your salt in the test, and so you will get a smaller percentage of salt in your final answer...which means a higher percentage of sand will be calculated. Of course if you didn't use the silver nitrate to test then this wouldn't be the case.
You should avoid splattering of the water solution. It is divined that it will also lead to loss of product as the salt that is in the water will be caught in the splatter and lost leading to a smaller measurement of salt in the end. So it is written.