Understanding 2 unit yield from 1lb ready to cook chicken
It's referring to the weight of each piece after cooking. 1 lb of uncooked chicken is 453.59 grams. Some use 450g as a quick rule as 4 grams is really not much in most applications. 450g divided by 2 would make each piece 225g raw. Cooked chicken generally loses 30% of its weight, so cooked is 70%, or .7 of the raw weight. 225g X .7 = 157.5g , which when rounded up is 158g. 

Keep in mind this is the approximate cooked weight of each piece, your milage will vary depending on the quality of the chicken, whether white or dark meat, and how much fat is on it.