You are right, for strength rating so close you wouldn't think there would that much difference in them.
So I was thinking about this... horizontal use pulling devices are rated with a 4:1 safety factor. so..
WLL Max strength
G70 4.7 18.8
G100 5.7 22.8
so the WLL is only 1k different, but the ultimate strength is 4k different. That strength is in a straight line pull. Once you start doing off-angle things the strength drops substantially. For example a 30-degree angle pull can reduce strength by almost 50%.
You are probably still exceeding the safe working load limits of even G100, and should consider upgrading the chain you are using to something with a higher rating than even the 5/16 G100.