Thanks for your interesting question about that unidentified old text. Here's my interpretation, although I can't vouch for the actual numbers cited.

Let's use an example. I like to backpack. I also like to eat well, so often my pack can be quite heavy. When it's time for a rest, rather than dropping my back to the ground, I try to place it on a waist-high boulder or fallen tree. That makes it much easier to put it back on, than bending over to pick it up off of the ground. Similarly, if my pack contained a 100 lb. bag of cement, I could never get it off the ground and onto my back by myself, but if a couple of strong friends lifted it onto my back, I could carry it--not happily, but I could do it.

May your burdens be light!

I don't know where this comes from, but my guess is that it is a rule of thumb of what people thought they could lift or carry. I think it works on the assumption that a person's legs are about three times as strong as a person's arms, and thus can generate three times as much force. This number is going to vary from person to person. It also does not take into account the fact that, as you point out, you can use your muscles for different actions.