I think its the consistency that gives manual laborers their strength. The average person might go to the gym for a combined 5 or 6 hours a week. But a farmer, brick layer, mechanic, builder etc is doing atleast 8 hours a DAY(often more especially with farm work) of intensive strength work. Yes it may not be the same as going to the gym and lifting very heavy for a few reps, but I think it strengthens you in a different way.
As already mentioned its the time under tension and extremely high reps that give the "farm strength". I also think that this sort of lower weight, time under tension, high rep training is better for strengthening connective tissue such as ligaments and tendons. Mainly because the varied work requires differing motions that may not be found in the gym. Kinda in the same way those big indian clubs are meant to be great for strengthening shoulders; odd rotating motion+ high reps of rotation+constant tension of holding them up+doing them daily....just something to think about