Weight Issues Etc. With A $5 And $10 Gold Liberty.

Greetings,
I'm new to the forum and somewhat new to collecting. I had a question concerning the weight of coins and how much room there is for leeway in regards to the official weight.

I checked out a few local coins from Craigslist, the ones mentioned in the subject, dates 1880 for the $5 and 1898s for the $10. I personally have a $10 Liberty which is slabbed so I was using that as a reference, and also brought a scale.

First off, this gentleman's $10 liberty was a bit strange looking compared to mine. It was cleaner, brighter color, but the details were somewhat "mushier" if that makes sense. Sorry I cannot post a picture. It was loose and not slabbed, but looked cleaner than my coin, and I guess "prettier", but not in an organic way. I'm not super experienced and don't expect a handled coin to look like an Ms61 graded coin, but I guess I'm just wondering if those are possible signs of cleaning. The man said it had not been cleaned.

But the most important issue was the weight. I weighed them and they were 16.33g for the $10 and 7.96g for the $5. The official weights should be 16.718g and 8.359g respectively according to my research. This is a difference of roughly .5 grams per coin.

Is this a normal amount for coins to be off? The guy told me the scale was skewed and cheap, but then I weighed a US quarter and dime, and they weighed 5.60g and 2.23 grams respectively, which is under the official weight, but well within .1g. I also weighed more regular dimes, nickels, quarters when I got home and they were all a little under, but again nothing equal to or more than .1g difference.

Also, mine was not an "s", and his was. I saw that it means SF mint, but what does that mean in contrast to one that isn't an "s" in terms of value?

Would coin cleaning have a corroding effect and drop weight, or being excessively handled cause that much of a drop in weight, or is there something off concerning about .45 to .5g difference and it could be a fake? He was selling them near melt value so I wasn't as worried about numismatic issues or cleaning, as I am still inexperienced in grading, but the weight issue weirded me out because I wasn't sure so I declined.

Any insight would be helpful, thank you.