There seems to be confusion when you use the quantity option with worn clothing. For example if you have a pair of socks and they are 32g each. I have switched it to total for pair and set quantity as one and now seems to calculate correctly.
Info/Explanation here..
https://github.com/galenmaly/lighterpack/issues/20
Ah cheers. From the explanation, I can kinda see what the developer's driving at.