Let's us an real life example: a women seeing her child get stuck under a car and in an instant without thought she is able to life up the car just for a moment to save her child. This should in theory be impossible since she should only be able to life 100% of her capacity +/- the standard deviation. So how did she managed to life 30-40 times her 100% lifting capactiy?
Without sadness the sad palm would still generate +/- 100% of his power since it's all backed from his inner power, however when he is extremly sad or suffient sad enough the sad palm would generate powers that exceed his 100% capacity just for a "moment" that would be enough for him to defeat someone of GWM caliber. Who else could have done the same thing in the end of ROCH?