If the rope was included with the anchor mate that might be the max load weight for the rope, Im sure some one behind a desk did some math that a 12 pound anchor + the resistance from pulling it up in the water might actually equal the weight of a 20lb anchor when lifting who freakin knows lol i'd put the 15 on an call it good it isnt that much of a difference and if it breaks you're only out 40bucks an lesson learned.