Are you charting tread depth for the sake of traction or treadwear? Obviously none of these are street friendly tires, but the Irok is famous for wearing out quick on the street. Since it has so much void between the blocks, there is less total rubber to wear through. Boggers are famous for having rather poor lateral traction, so off-camber on muddy trails is scary. Cutting TSL blocks in half is known to help them flex and gives you more biting edges, more similar to the selection of large radials available now. Of course, it also reduces the tread life since there's less rubber. For trail-only, the life of Swampers comes down to damage and age. Treadwear is more of a street-driven issue.
Also, for the sake of my own personal rant - I wish everyone buying new Swampers would return every out-of-round tire to Interco. Eventually they'd have to make round ones like every other manufacturer has figured out.
